Self-hosting a Git instance can be relatively straightforward. There are several options available, such as Gitea, Gogs, and GitLab, with Gitea currently being a top choice. Migrating the Bitcoin repository to a self-hosted instance would be manageable, although transferring issues and other related content might require some effort.
Developers would need to transition to a new domain and register, which may take about 5 minutes. While this could be a minor inconvenience, it's still feasible.
Nostr could play a role in this process as well. By having all developers on Nostr with a keypair, it's possible to verify their identities on another instance. An app could be developed for this purpose, or developers could simply include their npub in their profiles and sign something. This can also be done with ssh keys, and the new git verification mechanism.
Setting up mirrors as contingency plans could be a proactive approach to address potential concerns.