Surprised no one mentioned ForgeFed [0]:

> ForgeFed is a federation protocol for software forges and code collaboration tools for the software development lifecycle and ecosystem. This includes repository hosting websites, issue trackers, code review applications, and more. ForgeFed provides a common substrate for people to create interoperable code collaboration websites and applications.

It's based on ActivityPub [1], the same protocol that powers Mastodon [2], Lemmy [3], and Pixelfed [4].

[0] https://forgefed.org/

[1] https://activitypub.rocks/

[2] https://joinmastodon.org/

[3] https://join-lemmy.org/

[4] https://pixelfed.org/