Not sure how many other forums ("unclassified" in Latin, so won't touch 'fora'!) actually do allow an individual to delete a post, or even amend it, unless done within a (short) period of time. Under 90 minutes on the only one I have used which does offer that facility.
VVV "which I'm sure MNHQ are as keen to clarify" - can only say that pressure of work or whatever does seem to be making things far from "clarified" - eg on copyright being protected - after all this DM question has been running a month or so now.
While I can see the MN view about keeping threads intact, and avoiding deletions of single posts if at all possible, let alone mass deletion of all posts someone has made, I think there's a big question over how "open" MN needs to be.
On one hand, searching carried out on MN search page gets used a lot, and you need to be a member to use it, while searching with any web search engine (like Google) should take notice of a control file (robots.txt)
It's possible to implement all kinds of control over the internal search (like date limits, and blocking some subject areas through Hide/unHide) but no such control is made over external searching.
My guess is that MNHQ would not want to limit the searching from outside, as it means that they can be found by 'strangers', but that openness does mean a journalist / researcher for anyone outside can search for threads on most topics (if not all).
My feeling is that trying to date-restrict things isn't very easy, though it is technically possible (guessing at how I'd do it, would need every link to be examined and redirected to an "under X months old" section and an "over X months old" section) and an alternative, where individual threads could be 'public' or not, is again rather messy.
I'd be tempted to suggest MN have "public" areas where search engines like Google can look, and restrict access to any other areas by using the robots.txt file to prevent indexing, so while 'Site stuff' could be classed as public (or not!) there might be sections which could never be searched by Google (or other search engines, individually or all together sections of a site can be 'protected' from spidering aka indexing and thus from searches viewing those pages).
An option (so threads that MNHQ deem so useful as to be best in public) could be to make a copy of all posts in a thread and place it in a "public" section just for search engines to visit, to have a 'feel' for the variety of posts, without necessarily access to the whole site.
Well, it's late, I'm tired, and no doubt MNHQ would never go for any suggestion like this (because it would potentially limit the availability of 'strangers' coming to a thread on MN and looking around, with the possibility they later become a member) but it would perhaps give a degree of privacy (someone would need to register before using MN's search for hunting a particular quote, and having to register would put casual probers off, as they'd then be agreeing to MN's terms on what use they can make of anything they see).
Hmmmm... just checked and it looks like there's only a bland statement about copyright and nothing saying what use can be made of material on the site.
I'm pretty sure there used to be some comment about getting permission from MN to use any material, but that has vanished.
There's a bit for Media about paying 30 quid to be allowed to make a request to members, and then there are the actual Terms which describe only how someone submitting things is responsible for ensuring it isn't copyright or likely to harm (a formula for something) and so on, and there's a bit about contributions not being obscene, and about trolls, but nowt about how MN might use the material (apart from printed copies) where this latest change about "non-exclusive" copyright is in place.
Like FaceBook etc, MN accepts items, can edit them, and retains (non-exclusive) copyright on any submission, but doesn't seem now to have any clause to cover any other publication making use of items with permission
Is my memory going (hoped I'd have another 20-30 years before going dotty) or did it previously say something about "prior permission" ?
I should have saved the page (use "print" but not onto paper, I've stored things as PDFs for years so don't spend anything on paper or ink, and invoices are online, so no printing at my end, nor stamps or envelopes).