I’d be surprised if they don’t keep all of that. There were a number of sites for looking at deleted posts. They’d just go and grab everything and compare what was still there with what wasn’t and highlight the stuff that wasn’t there anymore.
Which is also possible here, though the mod log reduces the need for it. But if someone is looking for posts people change their mind about wanting anyone to see, deleting it highlights it instead of hides it for anyone who is watching for that.