Of course that’s a lot harder to do, since it’s a lot more than a UX change. So I’m not really slagging GitHub here. But it’s the right way to attack this problem.
In this case, I don't quite understand why stars and forks need to be deleted when switching visibility, since it seems like the two shouldn't be linked. I can star and fork my private repos just fine, so it seems like it was just easier to clear it out than to deal with RBAC at that level.
Many people won't notice some of their stars having disappeared, but many would be surprised to see their forks deleted without their consent, especially those that are not merely a mirror. People can have diverged forks after adding substantial changes without feeding them back to the original for a variety of reasons.
Pretend to do it, actually do it 24 hours later, and if they "undo" the delete within that time frame, just remove it from the delete queue process.
I still think you're right, but it's not even as simple as UX and small backend changes. Allowing the repo owner to re-play other peoples' actions comes with complicated policy decisions. GitHub could make it so only support can restore those, but now they're in the awkward position of having to say when it's okay to restore and when it isn't. Still, it may be the best compromise.