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.