Hacker News Reader

It uses an alternative backend where videos pre-removal had their dislike counters archived, and anything post-removal uses dislike data from users with the extension to crowdsource a likely dislike counter.

(in theory, if 5% of viewers of a video are people that have the extension, and only 10% of those viewers hit the dislike button, chances are that's a good sample size to assume that this video should have a 10% dislike ratio, but in practice it's probably a bit more complicated/weighted)

I think it used an alternate backend, possibly https://returnyoutubedislike.com/

Yes, it is that one.

YouTube still has a dislike button. They just don't show you the number of dislikes anymore.