As far as I know, I have rarely had posts or threads deleted.
Well - a couple of posts have been deleted where they contain a quote from an offensive post (with the offensive post being deleted and other posts containing material from the offensive post also being deleted). And have had a couple of TAATs deleted - but on self-reporting myself to Mumsnet.
I have found out about deletions when going to back to the relevant threads.
Do you think it would be a good idea for posters to be given an automatic notification when their posts / threads are deleted - ie so they know when they are overstepping the mark and can modify their posting accordingly? (Or would this result in some more 'robust' posters challenging the deletions and more work for MNHQ?).