Theory
Backups are usually out of scope, and usually full of the data
Retention and deletion policies often exclude backups for practical reasons. The exclusion is defensible. Leaving it unstated in the notice is not. Backups usually hold the data a policy says is deleted, so a privacy notice that is silent about them misleads the people it describes.
The tension
Claim: observation
Immutable backups help security and complicate privacy. The same property that stops an attacker from deleting data also stops the organisation from deleting it. This is a genuine engineering problem.
The research question is which architectures resolve it honestly, for example crypto-shredding with per-subject keys, and what those approaches cost in recovery complexity.