You can access data from deleted forks, deleted repositories and even private repositories on GitHub. And it is available forever. This is known by GitHub, and intentionally designed that way.
I don’t think you can create private forks from public repos (the fork is public upon creation). This is more like the opposite:
If there’s a private repo that is forked and the fork is made public, further changes to that original private repo become public too, despite the repo remaining private and the fork not being synced.
That is not exactly what they are saying. You could create a private fork of a public repo and the code in your private fork is publicly accessible.
I don’t think you can create private forks from public repos (the fork is public upon creation). This is more like the opposite:
If there’s a private repo that is forked and the fork is made public, further changes to that original private repo become public too, despite the repo remaining private and the fork not being synced.