❗️All Twitch content is now automatically opted in to Amazon generative-AI training by default, if you don't turn it off.

"If it was opt-in, nobody would opt-in," Twitch chief product officer Mike Minton said.

The off switch is web-only, buried under Settings, Security and Privacy. It won't stop your chat being used in streams that stayed opted in.

Opting out only covers your own channel. Chat in someone else's stream, and their setting decides whether your messages get used:
https://help.twitch.tv/s/article/twitch-account-settings?language=en_US#FAQ