well, auth is a little different as a client doesn't even have to see the nip11 they will receive the NOTICE and either be confused and crap out, or continue with an auth flow.. the relay will act accordingly by closing their stream till they auth, or sending them results that didn't need auth to gather, depends on the situation (kind4 or stuff like that). for payments required we got nip11 and, they will receive a "false" on their attempt to publish, with reason of "gotta pay". From the relay side, it's pretty clear, with nip11 and the various responses/notices, clients can either display these results or just silently get confused. Either way, the data is available in multiple forms (nip11 and response). At least, for my relays. I do override nip11 with custom info and use custom responses from a strfry plugin to be as informative as possible to the clients.