I don't think there are Twitch alternatives as clients on iOS given that they're very strict on what type of connections and functionalities an app can have, like they don't allow torrents, etc. I think they only have very few "prominent" software forks in App Store - if there are those forks, that is. Because those who submit apps to their store have to conform to many things like their policies, etc. before being published, if not the apps will then be rejected. For example, there aren't privacy-respecting clients on youtube in iOS unlike for Android, where you can use NewPipe instead of native youtube app.
For computers, there is Streamlink but I haven't tried that one. There is also GNOME Twitch but it's only for GNU/Linux OS.