Twitch stream embeds are not loaded when linked to from posts on Discourse installs using HTTPS. It seems like og:video:secure_url is not being preferred over og:video by Onebox, but I think it probably should be, at least on secure websites.
Thanks, guys. I realized that even if Twitch embeds loaded properly, they’d auto-play in posts. Auto-playing videos annoy the hell out of me, so I wrote a Twitch Onebox plugin to fix both problems.