And what does it mean for the transaction on the block chain if both the URL and the hash no longer match? Is it worthless now and unsellable? Or do you sell it with a note that says ignore the URL, ignore the hash, or both?

I did point out other issues and that may have been unnecessary, but a hash doesn't solve the rug pull problem if the art isn't part of the encrypted and (mostly) immutable transaction block.

Not really. The hash would prevent someone to pull the rug unnoticed, but it wouldn't prevent rug pulling in the first place.

With a hash, you would be able to prove that what's currently at that url isn't what you bought, but (since hashes are by definition non-reversible) you wouldn't be able to show or see what it was you bought (unless you stored it somewhere else yourself).

Which is usually trivial.