What does this have to do with public projects? This is entirely about private repos.
> Deleting a public repository
> When you delete a public repository, one of the existing public forks is chosen to be the new parent repository. All other repositories are forked off of this new parent and subsequent pull requests go to this new parent.
> Changing a public repository to a private repository
> If a public repository is made private, its public forks are split off into a new network. As with deleting a public repository, one of the existing public forks is chosen to be the new parent repository and all other repositories are forked off of this new parent. Subsequent pull requests go to this new parent.
[1]: https://help.github.com/articles/what-happens-to-forks-when-...