Earlier quoted context omitted.
This might be a bit of a weird question, but how do you remember which information needs to be deleted when you're at the point where you need to use backups?
How long are you keeping backups in cold storage for - usually you'd want maybe 6 months there, but no more, otherwise it's just going to grow unbounded and become a financial burden.
As far as I can tell to comply with a deletion request with absolute certainty requires infallible storage (which would remove the need for backups) or modifying backups (which contradicts the concept of a backup). Maybe you can claim 'force majeure' at some point, but perfect compliance seems impossible.