BsonClassMapSerializer used to be public but was made internal based on the assumption that it should be hidden from the user. But it turns out that there is a legitimate use case for directly instantiating a BsonClassMapSerializer (without registering it). If you want to write a custom serializer that delegates most of the work to a BsonClassMapSerializer but massages an intermediate BsonDocument during serialization/deserialization you need to be able to instantiate a BsonClassMapSerializer directly.