I like the approach that sourcehut took overall. I would like to find a way to implement issue tracking into Git, but the mailing list approach isn't a bad idea.
Maybe git-bug would work for you? https://github.com/MichaelMure/git-bug
> This is now more than a proof of concept, but still not fully stable. Expect dragons and unfinished business
FWIW I don't even use git-bug, I just know it exists :).
It's still not v1 where there would be some sort of backward compatibility guarantee, but it's close if you are using master. Note though that even without that guarantee, there is git-bug-migration to provide you some safety.
There is however some bugs and missing features, as in any software that young with an author busy buying/refurbishing an apartment ;)