GitHub’s repository architecture necessitates these design flaws and unfortunately, the vast majority of GitHub users will never understand how a repository network actually works and will be less secure because of it.

Trufflesecurity

Anyone can Access Deleted and Private Repository Data on GitHub ◆ Truffle Security Co.

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.