@perloid or Ken commented on one of the two latest monthly HPR Community News that putting things in issues (which is part of what a PR is, even if they are two different things in the GH interface) rather than a mailing list helps with keeping track on what's done and not.
I 100% agree with the observation that GitHub is problematic and GitLab is somewhat problematic. I would say though that GitLab includes a whole lot of things in the fully-free version, and what was missing last time I checked (literally years ago though) was basically things like integration with an existing enterprise authn/authz system. Gitea/Gogs does not have this issue, but not by having that functionality in the free version, rather by there not existing a non-free add-on, so I don't know if that is really an improvement for this particular issue.
As for distributed PRs and issue tracking, the only system I am aware of is git-ssb[0], which does it by living on top of an already distributed messaging system, secure-scuttlebutt[1].