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.
tl;dr - If a project has been forked or is a fork, you can bruteforce short commit id to see commits from other projects. It doesn't matter if those projects were deleted or made private.
Literally "it's a feature, not a bug". Absolutely hilarious that they don't think this is an issue. I can only hope that other sourceforges don't just copy this behavior "beCaUSe GiTHuB dOeS iTT!!!".