Do you happen to have or know of any good open source projects on any public platform like GitHub, gitlab or Codeberg, that uses Jujutsu/jj?
Checkout Gisia https://github.com/gisiahq/gisia
jj uses git as a backend. None of those need to "use" jj. You can use jj in an existing git repo, with your git wielding friends.
If I remember correctly, Mitchell Hashimoto uses it for Ghostty (probably under the git backend).
It's hard to tell since any given dev can just use jj locally, if they want to, while everyone else uses git.