> They would need a unique IP for every single repo
I don't think this is true. If your CNAME myghpage.example.com points to <somerandomstuff>.github.com which then routes you appropriately, all Github has to do is remove that <somerandomstuff> subdomain when you delete the page and that's it, myghpage.example.com will fail to resolve and you're safe.
This would work even if Github only had a single public IP address.
edit: The issue with Github pages is that they make you point your CNAME to something.github.io not something.youraccount.github.io, so anyone can recreate something.github.io after you delete it. If they simply created the pages as something.youraccount.github.io you would be safer, unless you lose your account name. That's why the random subdomain is preferable.