You can access data from deleted forks, deleted repositories and even private repositories on GitHub. In the video below, you’ll see us fork a repository, commit data to it, delete the fork, and then access the “Deleted” commit data via the original repository. The average user views the separation of private and public repositories as a security boundary, and understandably believes that any data located in a private repository cannot be accessed by public users.

Source: Anyone can Access Deleted and Private Repository Data on GitHub - Truffle Security Co.