In case it's of interest, here's the standard language from emails I send people:

We try not to delete posts that got replies, because doing so would be unfair to the other commenters in the thread. What I've done so far is reassign it to a random user ID, so it's as if you'd used a throwaway account to post it and there's no link to your main account. Does that work?

Sure, it is just a strange feature that me, being someone who has been posting here a long time, didn't know

As I just mentioned in a previous comment, I'd like to note how much I appreciate that you keep the contents of posts with replies. It's not only unfair to other commenters, it can also be unfair to readers. There's nothing worse than seeing only one side of a discussion, where the possibly-useful information was purged or edited into personal attacks on the other user. I much prefer SO-style attributed edits (though for a discussion forum this should be limited to mods, not arbitrary users), but time-limited editing with manual thought out exceptions as implemented here is a close second, far preferable to Reddit and other forums that allow arbitrary editing forever and only show a date (if that).