After checking the achievement notification, I was presented with this:
Context: I still have an account after using PDS to alter/remove all my posts and wiping my profile, but I still get messages from redditors seeking help so I check in from time to time.
After deleting your account, nobody can ever use that username again. (Including yourself)
For point 2, I’m not sure how you mean that but when you delete your account, it doesn’t delete your posts and comments.
I think their metrics showing people are deleting their accounts might be more impactful, because there are probably loads of unused accounts naturally.