Netflix Party at least has existed for years. I used it at least 4 years ago if not longer. What surprises me is that none of these sites have made a built-in version of this feature.

Well, and that's exactly the source of the confusion. I was sent a "Netflix Party" link recently and sure enough, it was a regular Netflix.com URL that loaded and played the movie we were planning to watch. I had no idea until I was told that I needed to switch to Chrome and install a browser extension before it would actually do the synchronized play.

If it was "Watch Together" or something, it would immediately clue me in that there was something else I was going to need to install or sign up for in order to make it work.

Regarding discoverability, can't Chrome extensions register themselves to handle custom schemas? If Netflix Party gave you a party://netflix/movie-id/session-id URL, then it would fail to resolve on a machine without the plugin installed, while still not requiring any centralized infrastructure.