diff --git a/src/libraries/System.Private.CoreLib/src/System/Utf8Extensions.CoreLib.cs b/src/libraries/System.Private.CoreLib/src/System/Utf8Extensions.CoreLib.cs
deleted file mode 100644
index ef0bc2105e1d3e..00000000000000
--- a/src/libraries/System.Private.CoreLib/src/System/Utf8Extensions.CoreLib.cs
+++ /dev/null
@@ -1,128 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System.Runtime.CompilerServices;
-
-namespace System
-{
-    public static partial class Utf8Extensions
-    {
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        public static ReadOnlyMemory<Char8> AsMemory(this Utf8String? text)
-        {
-            if (text is null)
-                return default;
-
-            return new ReadOnlyMemory<Char8>(text, 0, text.Length);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
-        /// </exception>
-        public static ReadOnlyMemory<Char8> AsMemory(this Utf8String? text, int start)
-        {
-            if (text is null)
-            {
-                if (start != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-            if ((uint)start > (uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-
-            return new ReadOnlyMemory<Char8>(text, start, text.Length - start);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="startIndex">The index at which to begin this slice.</param>
-        public static ReadOnlyMemory<Char8> AsMemory(this Utf8String? text, Index startIndex)
-        {
-            if (text is null)
-            {
-                if (!startIndex.Equals(Index.Start))
-                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
-
-                return default;
-            }
-
-            int actualIndex = startIndex.GetOffset(text.Length);
-            if ((uint)actualIndex > (uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException();
-
-            return new ReadOnlyMemory<Char8>(text, actualIndex, text.Length - actualIndex);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <param name="length">The desired length for the slice (exclusive).</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
-        /// </exception>
-        public static ReadOnlyMemory<Char8> AsMemory(this Utf8String? text, int start, int length)
-        {
-            if (text is null)
-            {
-                if (start != 0 || length != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-#if TARGET_64BIT
-            // See comment in Span<T>.Slice for how this works.
-            if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#else
-            if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#endif
-
-            return new ReadOnlyMemory<Char8>(text, start, length);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="range">The range used to indicate the start and length of the sliced string.</param>
-        public static ReadOnlyMemory<Char8> AsMemory(this Utf8String? text, Range range)
-        {
-            if (text is null)
-            {
-                Index startIndex = range.Start;
-                Index endIndex = range.End;
-
-                if (!startIndex.Equals(Index.Start) || !endIndex.Equals(Index.Start))
-                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
-
-                return default;
-            }
-
-            (int start, int length) = range.GetOffsetAndLength(text.Length);
-            return new ReadOnlyMemory<Char8>(text, start, length);
-        }
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static ReadOnlySpan<byte> CreateSpan(Utf8String text) =>
-            new ReadOnlySpan<byte>(ref text.DangerousGetMutableReference(), text.Length);
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static ReadOnlySpan<byte> CreateSpan(Utf8String text, int start) =>
-            new ReadOnlySpan<byte>(ref text.DangerousGetMutableReference(start), text.Length - start);
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static ReadOnlySpan<byte> CreateSpan(Utf8String text, int start, int length) =>
-            new ReadOnlySpan<byte>(ref text.DangerousGetMutableReference(start), length);
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static ReadOnlyMemory<byte> CreateMemoryBytes(Utf8String text, int start, int length) =>
-            new ReadOnlyMemory<byte>(text, start, length);
-    }
-}
diff --git a/src/libraries/System.Private.CoreLib/src/System/Utf8Extensions.cs b/src/libraries/System.Private.CoreLib/src/System/Utf8Extensions.cs
deleted file mode 100644
index 3249bb911ec985..00000000000000
--- a/src/libraries/System.Private.CoreLib/src/System/Utf8Extensions.cs
+++ /dev/null
@@ -1,295 +0,0 @@
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-using System.Runtime.CompilerServices;
-using System.Runtime.InteropServices;
-using System.Text;
-using System.Text.Unicode;
-
-namespace System
-{
-    public static partial class Utf8Extensions
-    {
-        /// <summary>
-        /// Projects <paramref name="text"/> as a <see cref="ReadOnlySpan{Byte}"/>.
-        /// </summary>
-        public static ReadOnlySpan<byte> AsBytes(this ReadOnlySpan<Char8> text)
-        {
-            return MemoryMarshal.Cast<Char8, byte>(text);
-        }
-
-        /// <summary>
-        /// Creates a new readonly span over the portion of the target <see cref="Utf8String"/>.
-        /// </summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static ReadOnlySpan<byte> AsBytes(this Utf8String? text)
-        {
-            if (text is null)
-                return default;
-
-            return CreateSpan(text);
-        }
-
-        /// <summary>
-        /// Creates a new readonly span over the portion of the target <see cref="Utf8String"/>.
-        /// </summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is null.</exception>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
-        /// </exception>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static ReadOnlySpan<byte> AsBytes(this Utf8String? text, int start)
-        {
-            if (text is null)
-            {
-                if (start != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-            if ((uint)start > (uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-
-            return CreateSpan(text, start);
-        }
-
-        /// <summary>
-        /// Creates a new readonly span over the portion of the target <see cref="Utf8String"/>.
-        /// </summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <param name="length">The desired length for the slice (exclusive).</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
-        /// </exception>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static ReadOnlySpan<byte> AsBytes(this Utf8String? text, int start, int length)
-        {
-            if (text is null)
-            {
-                if (start != 0 || length != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-#if TARGET_64BIT
-            // See comment in Span<T>.Slice for how this works.
-            if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#else
-            if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#endif
-
-            return CreateSpan(text, start, length);
-        }
-
-        /// <summary>
-        /// Creates a new <see cref="Utf8Span"/> over the target <see cref="Utf8String"/>.
-        /// </summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static Utf8Span AsSpan(this Utf8String? text)
-        {
-            if (text is null)
-                return default;
-
-            return new Utf8Span(text);
-        }
-
-        /// <summary>
-        /// Creates a new <see cref="Utf8Span"/> over the portion of the target <see cref="Utf8String"/>.
-        /// </summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is null.</exception>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
-        /// </exception>
-        /// <exception cref="InvalidOperationException">
-        /// Thrown if the resulting span would split a multi-byte UTF-8 subsequence.
-        /// </exception>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static Utf8Span AsSpan(this Utf8String? text, int start)
-        {
-            if (text is null)
-            {
-                if (start != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-            if ((uint)start > (uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-
-            // It's always safe for us to read just past the end of the string (since there's a null terminator),
-            // so we don't need to perform any additional bounds checking. We only need to check that we're not
-            // splitting in the middle of a multi-byte UTF-8 subsequence.
-
-            if (Utf8Utility.IsUtf8ContinuationByte(text.DangerousGetMutableReference(start)))
-            {
-                Utf8String.ThrowImproperStringSplit();
-            }
-
-            return Utf8Span.UnsafeCreateWithoutValidation(CreateSpan(text, start));
-        }
-
-        /// <summary>
-        /// Creates a new <see cref="Utf8Span"/> over the portion of the target <see cref="Utf8String"/>.
-        /// </summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <param name="length">The desired length for the slice (exclusive).</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
-        /// </exception>
-        /// <exception cref="InvalidOperationException">
-        /// Thrown if the resulting span would split a multi-byte UTF-8 subsequence.
-        /// </exception>
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static Utf8Span AsSpan(this Utf8String? text, int start, int length)
-        {
-            if (text is null)
-            {
-                if (start != 0 || length != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-#if TARGET_64BIT
-            // See comment in Span<T>.Slice for how this works.
-            if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#else
-            if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#endif
-
-            // It's always safe for us to read just past the end of the string (since there's a null terminator),
-            // so we don't need to perform any additional bounds checking. We only need to check that we're not
-            // splitting in the middle of a multi-byte UTF-8 subsequence.
-
-            if (Utf8Utility.IsUtf8ContinuationByte(text.DangerousGetMutableReference(start))
-                || Utf8Utility.IsUtf8ContinuationByte(text.DangerousGetMutableReference(start + length)))
-            {
-                Utf8String.ThrowImproperStringSplit();
-            }
-
-            return Utf8Span.UnsafeCreateWithoutValidation(CreateSpan(text, start, length));
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        public static ReadOnlyMemory<byte> AsMemoryBytes(this Utf8String? text)
-        {
-            if (text is null)
-                return default;
-
-            return CreateMemoryBytes(text, 0, text.Length);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
-        /// </exception>
-        public static ReadOnlyMemory<byte> AsMemoryBytes(this Utf8String? text, int start)
-        {
-            if (text is null)
-            {
-                if (start != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-            if ((uint)start > (uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-
-            return CreateMemoryBytes(text, start, text.Length - start);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="startIndex">The index at which to begin this slice.</param>
-        public static ReadOnlyMemory<byte> AsMemoryBytes(this Utf8String? text, Index startIndex)
-        {
-            if (text is null)
-            {
-                if (!startIndex.Equals(Index.Start))
-                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
-
-                return default;
-            }
-
-            int actualIndex = startIndex.GetOffset(text.Length);
-            if ((uint)actualIndex > (uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException();
-
-            return CreateMemoryBytes(text, actualIndex, text.Length - actualIndex);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="start">The index at which to begin this slice.</param>
-        /// <param name="length">The desired length for the slice (exclusive).</param>
-        /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>
-        /// <exception cref="System.ArgumentOutOfRangeException">
-        /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
-        /// </exception>
-        public static ReadOnlyMemory<byte> AsMemoryBytes(this Utf8String? text, int start, int length)
-        {
-            if (text is null)
-            {
-                if (start != 0 || length != 0)
-                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-                return default;
-            }
-
-#if TARGET_64BIT
-            // See comment in Span<T>.Slice for how this works.
-            if ((ulong)(uint)start + (ulong)(uint)length > (ulong)(uint)text.Length)
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#else
-            if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
-                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
-#endif
-
-            return CreateMemoryBytes(text, start, length);
-        }
-
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target <see cref="Utf8String"/>.</summary>
-        /// <param name="text">The target <see cref="Utf8String"/>.</param>
-        /// <param name="range">The range used to indicate the start and length of the sliced string.</param>
-        public static ReadOnlyMemory<byte> AsMemoryBytes(this Utf8String? text, Range range)
-        {
-            if (text is null)
-            {
-                Index startIndex = range.Start;
-                Index endIndex = range.End;
-
-                if (!startIndex.Equals(Index.Start) || !endIndex.Equals(Index.Start))
-                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
-
-                return default;
-            }
-
-            (int start, int length) = range.GetOffsetAndLength(text.Length);
-            return CreateMemoryBytes(text, start, length);
-        }
-
-        /// <summary>
-        /// Returns a <see cref="Utf8String"/> representation of this <see cref="Rune"/> instance.
-        /// </summary>
-        public static Utf8String ToUtf8String(this Rune rune) => Utf8String.CreateFromRune(rune);
-    }
-}