You can not delete a temporary fork of a repo, which you created as a part of an open security advisory, if you close the advisory without making the security advisory public. This will include cases, where to chose not to disclose the advisory, or the advisory was not valid and could be closed. Regardless, if you created any temporary forks as a part of it, they will only be delete after you make the advisory public.
There is no documenation clarifiing this.
Once you do make the advisory public and close it, you should be able to delete any provate forks created.
GitHub support has reached out to me, I am awaitng for a reponse after clarifying the above details.