Crypto-shredding does not delete anything from your old backups

An erasure request comes in. The person's data is in your database, in three replicas, and in nightly backups you keep for five years because a regulator told you to. You cannot rewrite a backup. Most teams answer with crypto-shredding: encrypt every record under its own key, and when the record must go, delete the key. Ciphertext without a key is...