iPhotos > Shared Photo Albums: Protect from Remote Deletion
If the person who shared the Albums either deletes them, or their account is otherwise deleted, your access to the shared albums will also cease. However, if you want to ensure future access to the shared pictures, you must copy the pictures locally to your own iPhotos account. 

In iPhotos:


Click on Shared Album
Click "Select"
Choose all the photos you want to protect from the sharer deleting the Shared Album (even or losing their account completely)
Click "Share" button
Choose "Save Image"


Now if the person who shared the album deletes it, you've copied the images into your own iPhotos account.  In the age of this global pandemic, I suspect a lot of cherished family photos will be lost forever as the virus claims loved ones who obviously won't be paying the monthly iCloud storage subscription fees.

Hope this helps folks safeguard their cherished memories from inadvertent loss-