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.
If my thing was public in the past, and I took it private, the old public code is still public.
That's... How the Internet works anyway.
Edit: See Eager Eagle's better explanation below.
TL;DR - be careful who you allow to fork your private repos. And if you need to take a public repo, which has forks, private, consider archiving the repo and doing all the new work in a new repo. Which is arguably the reasonable thing to do anyway.
Still a misleading title. This isn't a way to break into all or even most of your private repositories.
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.