If GitHub does get sold to MS and I end up moving to GitLab, I'll probably push one last commit to the GitHub repo adding a header saying the project has moved, with a link to the GitLab repo. It's not perfect, but it wouldn't be too bad.

Until MSFT/GitHub does what Sourceforge.net did - taking over project sites from projects which moved away and adding malware (adware/spyware) into those ;)

(I believe with all critique on Microsoft they aren't as bad, but want to exemplarize the risk)

It might just be cleaner to close the github repo and when other projects find a 404 where it used to be, they'll have to use super detective skills (i.e. Google it) to find the project's new home. And if they can't find it that way, then nothing of value was lost. (Yes, yes, I know it's more nuanced than that, but if you wanted permanence, you'd be hosting on your own domain, right?)

This would be a great way for them to accelerate migration away from Github.

I expect Google could be convinced to accept certain files or metadata in a README as equivalent to a 301 permanent redirect, meaning searches will remain effective. That would account for a lot, especially if Chrome begins to honour it.

Actually, the #1 issue is that everyone can easily file an issue/contribute in other ways at Github without having to create another account to do so.

You can sign in with your Github account to gitlab.com.