Hello!
I would see 2 ways to do it based on your screen capture:
1/ Use the "Date" on the last column to set the day of the last time you watch a specific title
=> Then you can list your titles by date, the oldest watched (or missing a date) will show up at the end of the list
https://www.lddb.com/collection.php?action=list&sort=dateor
2/ Use the "Notes" to add "not watched yet"
=> You will then be able to search inside your collection for "not watched yet" only.
https://www.lddb.com/collection.php?action=list&collsearch=not+watched+yet&sort=titleJulien