It's part of the protocol, but many clients don't implement it correctly.
Re:Easy XMPP: What are we doing here?
by jimktrains2 on Thursday January 19, 2017 @04:10PM
(#13439526)
Multi user chats/chatrooms are part of the protocol though.
Or it's part of the protocol, but design decisions lead to client interoperability problems?
Maybe there's a third answer, but both of these are problems with XMPP.