Deletion requests affect monthly backups as well, although it's sort of up in the air how many months the EU will accept as a reasonable business practice for keeping should-be-deleted user data.

The best way to delete user data is to encrypt it all with per-user keys so they can be shredded immediately upon request. Backups of user keys themselves can be very short-lived since the keys are static and the lifetime is scoped to GDPR/other laws. Then the encrypted data can be archived indefinitely, so long as encryption keys are never stored in those archives.