Depends on which country's GDPR authorities you ask. At one point the French authorities said you don't have to delete data from backups, the Danish authorities said you have to delete when technically feasible, and the UK authorities said you had to put the data "beyond use" which has been interpreted to mean that if you ever restore from the backup you have omit the "deleted" data.

My guess is that most places go with not taking any active steps to delete things from the backups themselves, counting on media rotation to eventually overwrite the data. When restoring they omit anything that is on the "should be deleted" list.