Can project management features not be made part of a dumb repo on the db side? (Spoiler: yes, and many projects have explored this — setup unfortunately has never been as easy as "we'll invite u to the gh, check ur email".
Perhaps with decentralization push of web3/QR etc, we'll get over the hump.
I think that parent means also things like CI, release repository, PR review, etc.
These are not easily portable, but honestly is because of this lock-in that I prefer to use separate/independent tools. For my open source project [0], I am putting things on github and it is the link that give to most people, but in reality is just a mirror to the gitlab repository[1], which I use for CI and static page hosting, and the "project management" is done on Taiga [2]