Reminder to all OSS projects: it is extraordinarily easy to setup a simple CI job to keep your code in sync between multiple Forges. And getting email notifications from a second Forge is 0 extra effort. At least give people the option to start moving away from GitHub to contribute to your project. It will, ultimately, be better for the ecosystem.
The difficult part is all what's around the code:
* the tickets/PR (including the closed ones)
* the links referencing the project
* the CI setup
* for large projects, the committers permission setup
* if applicable, the push/commit/branch rules
All that will be deeply annoying to migrate on a per project basis, or might get lost.
But that's not even the worst on my opinion. Losing the go-to platform for finding software is (fediverse for software when?).