Could Github developer workflow metadata (e.g. issues) be exported/serialized into a git repo for decentralized replication and/or import into an alternative?

Yes. I built a tool for that a few months ago: https://github.com/simonw/fetch-github-issues

I have it running in one of my repos using a GitHub Action that's triggered when an issue is created or updated - it commits a JSON export of that issue back to the repo.

Then I can git clone the repo and get all of the issues data.

not to git but there are bots to export to ie gitlab