I would expect Git forges (and other configuration control adjacent software) to be the easiest services to federate. Git is already decentralized, and the usual concerns regarding encryption and privacy are essentially irrelevant since everything is meant to be public anyways.

If you ask me it's strange that the federated social media (which leaks PII everywhere) and federated messaging (which is a security nightmare) have taken off but decentralized Git forges don't exist. My best guess is that the people who care are already using mailing lists?