Recovering deleted photos from a shared album on iPhone

Shared Albums don't count against your iCloud Storage, because they are lower quality copies of the pictures in your Library--the don't have all the same information. Shared Albums don't replace the pictures in your Library-- they are completely separate things. As muguy said, pictures deleted from your Library go to Recently Deleted for 30 days (or until that safety is defeated.) Picture removed from Shared Albums are gone.

It is valuable to keep backups of pictures and important data on an external drive.

You'll need to add the photos again from your library.

Items that you delete are moved to the recently deleted album. You can restore from there. If you further delete from that album, the photos are permanently deleted and irrecoverable.