https://wccftech.com/how-to-get-deleted-whatsapp-media-back/

There are plenty of times when you delete media from your galleries stored by apps like WhatsApp because you are running out of storage only to find out later that you need to the check that image again. It appears that WhatsApp can now download them again for you even if you have deleted the images locally on an Android phone.

“Now, when you delete media from your phone’s storage (through the gallery or a file manager, not from WhatsApp itself), you can go back to the conversation in WhatsApp and you’ll find the blurred thumbnail there with a download button again,” Android Police’s Rita El Khoury reports. You can simply tap on the blurred image to download it again.