My understanding is that the GDPR “right to be forgotten” does not cover backups. There may be some exceptions, but there are practical limits on its reach.
Anyway, think about the spirit of the law, and then think about how that interacts with backups. If someone asks to be deleted from your system, you do so, and then you restore a backup with their data, you have clearly violated the intent.
The GDPR contains exceptions for data storage for which it is infeasible or outside reasonable effort to delete individual records or you have legal compliances to uphold.