What happens to the forks when deleting the original repository?

It depends whether the original and the fork are public or private, according to GitHub's documentation:

  • Deleting a private repository will delete all of its forks.
  • Deleting a public repository will not delete its forks.

More detail on this page:

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.

  1. If original private repo deleted: all of its forks will be deleted.

  2. If original public repo deleted: its forks will not be deleted.

  3. If public repo is converted to private, and deleted the original: forked repo will not be deleted.

    a public repository's forks will remain public in their own separate repository network even after the parent repository is made private.

So, If you fork from a public repo, forked repo will not be deleted. If you fork from private repo, forked repo will be deleted.

Source: https://help.github.com/en/articles/what-happens-to-forks-when-a-repository-is-deleted-or-changes-visibility