Could you provides a way to opt-in to case-sensitive property deserialization?

It could take the form of a property of JsonSerializerSettings or JsonSerializer. This property could be a boolean or an enum type (permits a fine grain choice: in ctor and/or properties).

JsonPropertyCollection.GetClosestMatchProperty is used in:

• JsonSerializerInternalReader where is already present a property Serializer of type JsonSerializer,
• DefaultContractResolver whose instance is provided by JsonSerializer.