This article explains how deleting your repository or changing its visibility affects that repository's forks.
Warning: If you remove a person’s access to a private repository, any of their forks of that private repository are deleted. Local clones of the private repository are retained. If a team's access to a private repository is revoked or a team with access to a private repository is deleted, and team members do not have access to the repository through another team, private forks of the repository will be deleted.
Deleting a private repository
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.
Private forks and permissions
Private forks inherit the permissions structure of the upstream or parent repository. For example, if the upstream repository is private and gives read/write access to a team, then the same team will have read/write access to any forks of the private upstream repository. This helps owners of private repositories maintain control over their code.
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.
In other words, a public repository's forks will remain public in their own separate repository network even after the parent repository is made private. This allows the fork owners to continue to work and collaborate without interruption. If public forks were not moved into a separate network in this way, the owners of those forks would need to get the appropriate access permissions to pull changes from and submit pull requests to the (now private) parent repository—even though they didn't need those permissions before.
Deleting the private repository
If a public repository is made private and then deleted, its public forks will continue to exist in a separate network.
Changing a private repository to a public repository
If a private repository is made public, each of its private forks is turned into a standalone private repository and becomes the parent of its own new repository network. Private forks are never automatically made public because they could contain sensitive commits that shouldn't be exposed publicly.
Deleting the public repository
If a private repository is made public and then deleted, its private forks will continue to exist as standalone private repositories in separate networks.