In the old wire protocol, the server responded to all wire protocol messages with an OP_REPLY wire protocol message.  With OPM_MSG, the server now responds with OP_MSG.  The only remaining driver functionality that requires OP_REPLY handling is the initial handshake, which is still initiated with OP_QUERY.

However, the driver still has code in both ReplyHeader and ReplyMessage that handles the full range of OP_REPLY responses.  That code is unused except in tests and can be removed.