So this task works well, thank you… but the whispers are simply collapsed and still viewable by Admins and moderators when expanded.
This does tidy up topics, but I was hoping they could be completely removed in their entirety. I wondered if the whispers could all be moved to a new topic, and that then deleted or hidden from mods at least, but then I found this: Is there a way to remove all the deleted/hidden posts? And more recently: Why is it not possible to permanently delete topics or posts?
Unfortunately the above suggest there is no way to permanently remove the data 