Roku used to have an unofficial twitch app that was removed, then they had the official twitch app which was never updated and works off and on, but it’s a pretty bad app. Then there was a app called Twitched and Twitched Zero which worked great, the former was 1$ and worked flawless but Twitch served the author with a cease and desist letter so that got removed.
Now we have a new app on the scene called TTV Stream. It’s actually pretty amazing, very robust. You can watch past broadcasts, follow channels, search games, view chat, and more.
It’s an unofficial roku app so you’ll have to add it manually via a computer or phone. You can’t search for it from a roku device. I also would recommend creating a new twitch account to use this app just in case anything nefarious happens. The app requires some permissions that could be abused. Time will tell, but so far I am really liking TTV Stream.