So what if Github itself is not OSS? The technology behind is already open source and decentralized. You can switch to a Gitlab instance with just a few mouse clicks while also migrating your issues and pull requests.
I don't believe issues or pull requests are part of the Git repo. If there is a way to migrate them it would involve some GitHub API which could be shut down.
as much as many projects could use a clean slate here, i dont think the burden of migranting them manually or the act of just dumping them is realistic.