Backups are not archives
I think that most people miss the key distinction between backups and archives. As other comments point out, backups are generally not indexed at a granular level. Archives are.
Backups are an operational tool that allows for recovery in a disaster. As such, there should be no need to keep backups for longer than you would need to recover from in such a disaster.. say a month or two.
Archives are generally kept online and provide fast access to specific records.
If this regime is followed then the right to be forgotten is simple to implement. You delete or mask records in the archive and wait for backups to expire. And the records are gone in the 1 to 2 month period.