Despite a successful beta launch on Android, Valve's game streaming service Steam Link won't be coming to iOS, at least not anytime soon. According to the official Steam Database Twitter account, Apple rejected the Steam Link app over apparent “business conflicts with app guidelines.”
It's not immediately clear what the "business conflict" here is. Perhaps, it may be due to the fact that the app allows an iOS user to access Steam, which is another app store by itself. You can check out Valve press statement on the Steam Link app for iOS being rejected by Apple here.