Tip: WhatsApp can re-download deleted media, hide it from your gallery
First, and this was pointed out by our tipster Nitin, is the fact that WhatsApp can re-download media that was deleted from the device's storage. I can't say how new (or old) this is. It's been years since I've deleted some WhatsApp media, but a friend did so several months ago and asked me how to get them back. The option wasn't there then. So the best I can guess is that it's relatively new-ish.