The namespace contains 10 classes. Outside of tests, only JsonMessageEncoderFactory is used — and even then only within the namespace itself (CompressedMessageJsonEncoder calls it internally).

 The full picture

 - JsonMessageEncoderFactory is the single entry point — it implements IMessageEncoderFactory and is the only way to reach the concrete encoders. But even this factory is never instantiated in production src/ code outside the namespace.
 - The only non-test, non-namespace reference is in tests/MongoDB.Driver.Tests/Core/Helpers/MessageHelper.cs (TranslateMessagesToBsonDocuments), which is a test helper. However it looks wrong to use the separate encoder to test the actual encoder behavior/output. The helper method should be rewritten to go through binary encoding instead - serialize via BinaryMessageEncoderFactory into a MemoryStream, then deserialize back to BsonDocument.
 - All 10 classes are internal, so they're invisible outside the assembly entirely.

Bottom line: The entire JsonEncoders namespace is only consumed by tests. It has no production call sites outside its own directory.