I'm kind of surprised this isn't possible on Twitch's TV apps. The search function even says "find your favorite stream, category, or tag", but it doesn't search for tags at all. If I search "speedrun" for example, I can find channels or stream titles that literally contain the word "speedrun", but not the speedrun tag. For that matter, I'd like to be able to follow a tag in the same way I can follow a game category, but I suppose that's out of the scope of this request.