I cannot envisage any circumstances in which it can be right to retrospectively delete a thread, as opposed to deleting particular posts which might be libellous or disclose stuff that shouldn't be disclosed, such as private information.

Some forums when deleting posts leave the "name" of the poster but indicate the post has been removed. That seems like good practice to me.

A meta discussion as I understand it is, for example, when we back off from an argument, and try to explore why it is it is getting heated. For example in a domestic setting you realise that the main reason you are rowing might be because your blood sugar is getting low, and you need a bite to eat.. Going meta can be very helpful, though we need to recognise that it can be seen as infuriatingly evasive. In the Mudcat context it can make a lot of sense.