It's still there, but you need to go to your own profile and tap general settings. The auto-play option is in there. I think you did use to be able to change it in the settings on the video, but not anymore.
I don't think it's possible to get rid of auto-play completely because a lot of countries have laws against it for minors. I'm pretty sure it's still turned off by default for accounts owned by people under 18.
"It's Dyin' time."
Captain Bruce Diehard,
Die Hard