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.
While this is still a massive problem, it does require a public fork at some point. So if you have a private repo that has never had a public fork, you should be safe.
While this is still a massive problem, it does require a public fork at some point. So if you have a private repo that has never had a public fork, you should be safe.