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.
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.
So is much of the technology behind Discourse.[1]
[1] https://discord.com/open-source (click on "OS Libraries")