Backups usually have limited retention after various data protection agencies. In my current location the recommended time is a few weeks for backups, a few days for logs etc. Those agencies are fully aware that nobody goes around purging data from backups (which may also be hard with airgapped backups etc.) and you might need to wait before being "forgotten".
Are they allowed to restore the account from backup if the user asks for the reopening of the account?
You only need to state to what extend and period the information is stored in backups, and have a process in place to purge it again in case of a restore.
I never realized that was the rule. I thought you had to go in and wipe the backups regularly.