Agree with you it's a legal issue. If I just drop the word delete and instead say just archive (out of mailbox) or just auto-label and stop at that, we're back to where I'm thinking about. Let's say we have to keep everything at least 7 years and we only need to delete some certain stuff at that time.
We might let users keep their other data longer than 7 years out of convenience to them. But then they reach their quota. Now here we are with this topic where we can safely discuss deleting any of the remaining content that is 7+years old and not the stuff that was auto-deleted based on legal/compliance policies.
Just looking for ideas and we need to be able to think about it technically even though legal hold the real important answers. I guess I'm just narrowing the scope of my comments to just safe-to-delete data, per legal/compliance teams' policies.