I'm pretty sure that wouldn't happen as we would have heard of it by now (like if someone maintains a fork for years, then suddenly their repo vanishes because

I'm pretty sure that wouldn't happen as we would have heard of it by now (like if someone maintains a fork for years, then suddenly their repo vanishes because the parent went private), but that would be interesting to check what exactly happens. Maybe the text "forked from x" is simply removed from the children repositories.