Playing the devil’s advocate a bit further here…
You have CLEARCHAT for success and the NOTICE for failure. CLEARCHAT will carry over the message ID if you sent it with the timeout and the NOTICE message should be good enough for the user to identify which one didn’t work.
None of them apart from NAMES (which is only a reply to JOIN). If they’re too long, they simply break the IRC max length (you requested the Twitch-specific capabilities, should be able to handle it). Try /mods in #dota2lobby for example.