It's hard to tell since any given dev can just use jj locally, if they want to, while everyone else uses git.

All my prs are named phil-<hash> and have been for years now. I keep waiting for someone to care/notice, but no one has.

Technically, you can still tell actually - jj writes a "change-id xyz..." in the git commit object header, which remains there as it's pushed around. It's just typically not made visible by regular things. (I wonder what other random garbage has been hidden in git commit headers that noone has seen)