The flag was there for quite a while, i don't remember exactly but probably a year, it works, but not on all websites, but I've tested it on YouTube and news websites. I think (and i hope) what they are trying to do is to make it work on all websites.
in my opinion, autoplay blocking is not enough if it doesn't block video preloading. when I set autoplay blocking to block, it stops YouTube videos from playing automatically, but the website still downloads the video in the background, waiting for me to hit the play button. this consumes unnecessary internet bandwidth.