If a data subject requests their data to be erased, you should remove their data from active systems so that it is no longer being processed, but you don't have to remove it from backups or other passive systems. You should however store some sort of marker so that if you need to restore data from backups, the data subject's data will be re-erased or otherwise stopped from entering active systems again.
And if a data subject asks, you have to tell them how long you store your backups of their personal data.
I think that's perfectly reasonable. And if your backup retention policy is "forever", now might be a good time to re-evaluate that policy.