I like the idea of closing the gap where GitHub might fail, and I love ActivityPub-based services and the federated web, but I think this isn't the best solution. Git is already decentralized to the extent necessary to be useful for projects. An open source project will generally have one and only one canonical upstream location. If that location needs to move, git is accomodating.

The better strategy is to work on encoding ticket tracking and such into git objects, in my opinion.