Ok, looking at that thread from July (!) - it seems this was never resolved, and this paragraph
In the case of PMs, messages by poster A can often contain personally identifying information about poster B. Because we’re putting poster B’s right to delete her data above other concerns, when poster B deletes a PM that was sent to her, the PM is deleted from our servers and will disappear from poster A’s ‘sent’ folder .
Likewise, if poster B de-registers or asks us to delete all her data, we think she will rightly expect PMs between herself and poster A to be deleted from our servers. That means they’ll also disappear from poster A’s ‘sent’ folder.
Does not address this problem, posed on that thread
If the pm is deleted from your servers at the same time as the sender deletes it, can you please explain how you will retrieve that to use as evidence if there is a case of harassment or abuse?
MNHD stated
we need to privilege the right of users to delete their data. - which is one thing, but surely they also have a duty to mumsnet users who are being abused by random people who log on for the night and then de register.