Postel's Law says: be conservative in what you do, be liberal in what you accept from others.  So the client should not violate the RFC when sending data to the server.  The server, on the other hand, by that law could accept "dirty" data if it can do something reasonable with it...except that in general the IMAP RFC in particular says "don't do that".  IMAP servers are supposed to be IMAP RFC sticklers, even when accepting input.  So gmail is broken for some definition of broken, and so is imaplib.

Since the code has been this way for a long time, and gmail does in fact accept it, I think a doc warning about []s violating the RFC should be sufficient; a warning would probably just be annoying to little real benefit.