How many applications are left that still use Mercurial? I can only think of two big ones, the JDK and Mozilla's entire code base. Are there still others around? Seems like everyone's moving to Git these days, which is great considering I never did get the hang of Mercurial.
Facebook?
It's quite a bit different from stock hg at this point. The server is custom, and decent chunks of the CLI have been rewritten in rust.
https://github.com/facebookexperimental/eden
Previous discussion https://news.ycombinator.com/item?id=23124095