Most of the channels I work in have 1k+ chatters so all the “normal” IRC MODE stuff just doesn’t work, you should be reading TAGs instead
The documentation no where says you can send MODE messages.
The main Dev Documentation do not cover the available chat commands, that is covered in the general help guide (for everyone not jsut devs) here
https://help.twitch.tv/s/article/chat-commands?language=en_US
These commands can be sent a /command or .command to avoid IRC library conflicts that interrupt / and sends as a command instead of a PRIVMSG