Often, I want to follow a streamer only for certain games/categories they stream, such as following artists only for their Art and Just Chatting streams but not their gaming streams, or I might just not want to see anyone playing certain games, no matter how much I like that streamer otherwise (e.g. to avoid spoilers or just because I have no interest in it). Notifications for streams I have no interest in just distract and annoy me, and lead to me either unfollowing the streamers or ignoring them so much that Twitch stops sending them.

While I think that opting in to categories would be too harmful to streamers and discourage them from trying new things, I think it would be great if viewers could opt out of notifications for specific categories. I think the fairest and simplest way (UI-wise) would be to have it as a global setting, so that you could avoid notifications for categories you're never interested in seeing.