Interesting that their issues are blamed on "dual SCM", not on Mercurial itself. I guess just the weight of contributors expecting Git as the default is sinking the big Mercurial projects these days.
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?