I'd second this as a feature request. Ideally we'd like a second retention policy to prune removed items after X days from the backup. Perhaps it's just a simple checkbox on the retention policy "Purge removed items after X days" or something. Our only option right now is "keep forever" and it's eating a lot of storage, and our users understand deleted items have to be recovered in a certain amount of time.
The way I understand it if I do a one year retention for backups, and a user keeps an item for longer than a year if they delete it, it could be gone forever since it's no longer retained in the backup. We essentially want our backups to mirror a user's mailbox with a retention of about 30 days for items that no longer exist regardless of how old that item was when it was removed. That would be a keep forever retention policy, but a second option to prune removed items after a certain amount of time.