The IRC gateway passes whispers using the WHISPER code:

[16:36:15] [@] @badges=bits-charity/1;color=#3E2E18;display-name=foo;emotes=;message-id=2;thread-id=21001676_470220570;turbo=0;user-id=2109996;user-type= :foo!foo!foo.twitch.tv WHISPER myuser :this is a whisper

But I am unable to find any reference to that code on https://dev.twitch.tv/docs/irc/commands where I would expect it, or any other documentation page under the "Chatbots and IRC" section.

I would suggest adding the WHISPER command to the list of commands at the top of the page on the IRC: Commands section, something similar to:

WHISPER Receive a Whisper message from another user


and then the prototype at the bottom, to include the fact that it appears the WHISPER command is (appears to be) a one-way command (the server can send a WHISPER to you, but you can't send a WHISPER to other users. (When I tried to send a whisper, I got: [16:45:58] tmi.twitch.tv reported an error: WHISPER :Unknown command).

Or maybe this is deprecated or not final yet, but I figured I'd point it out just in case it was an oversight. Thanks for all the hard work!