Even if the site is not in SSL, nothing forbids to display SSL videos.
No problem at all!
Except that it's really not.
Aside from Twitch itself saying very clearly on
https://dev.twitch.tv/docs/embed that it only works on SSL (because their API won't allow it), the reality is that this is going to become more of a problem going forward, especially as tech like CSP becomes more mainstream.
Better to get with the program now and just implement SSL. Besides, it's protecting your members against types of attack to have that - SMF _tries_ but not 100% successfully to protect your password as you login, but only SSL actually does do so from, say, wifi observers.