The implementation of the legacy API uses the legacy wire protocol (OP_INSERT/UPDATE/DELETE) on all server versions. To ensure that it continues to work with 5.1+ servers (which no longer support this protocol), the implementation should be updated to use write commands and OP_MSG as applicable. This can probably be accomplished by using BulkMixedWriteOperation, as the new CRUD API implementation does, and possibly writing a small adapter for the exceptions and result.