They had a release just a few days ago. It's definitely not abandonware.

It’s still used by Meta, at any rate (albeit a very scaled version thereof). Meta picked it for their monorepo when Linus wasn’t willing to play ball on extending Git for their use case.

Is it still used there? I know they did in the past, but reading up a bit on the background on all of this I found https://github.com/facebook/sapling, and it seems that's what they're using now?

I don't know for sure today, but Sapling was mercurial-compatible, at least early in development.