I should disclaim this with: I've only spoken to people who have looked into this. I can't guarantee it's correct, but it's the situation as I know it currently. It's also assuming cold backups that are not processed in any way that accesses the should-be-removed data
You don't need to remove data from backups in realtime but you do need a system in place to re-delete data after a backup restore that previously had a removal request made against it.
From what I've heard so far the best method is some form of GUID that means nothing when there's no data attached to it but can be used to identify user records that need removing if they re-appear during a backup restore
Naturally, you'll want to make the please-delete-these-GUIDs table more redundant than the rest of what you've backed up
You don't need to remove data from backups in realtime but you do need a system in place to re-delete data after a backup restore that previously had a removal request made against it.
From what I've heard so far the best method is some form of GUID that means nothing when there's no data attached to it but can be used to identify user records that need removing if they re-appear during a backup restore
Naturally, you'll want to make the please-delete-these-GUIDs table more redundant than the rest of what you've backed up