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.

I tried to contribute a few years ago. The mercurial clone was taking multiple hours. They already had an non official git, which took 15 minutes to clone.

Isn't mercurial abandonware? Or maybe I'm just remembering that gitlab dropped support. If it's not dead yet seems to be getting there

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.