Doesn't GitHub delete forks if the original is deleted? Of course, actually cloning the fork onto your computer and pushing it to a new repository without using GitHub's "fork" feature should get around this easily.
When you delete a private repository, all of its private forks are also deleted.
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.
> 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.
When you delete a private repository, all of its private forks are also deleted.
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.