It seems that Google goes to great lengths to prevent you from doing this with each new iteration of the YouTube platform. Depending when you read this, you may have to search for a new answer. As of April 2021, the only working answer is to follow the above tutorial here by Keagan C.

After that, you'll probably want to delete the Watch Later videos by doing the following: go to your Watch Later list, press Ctrl + Shift + J if you're on a PC, or Cmd + Option + J if you're on a MAC.

Scroll to the bottom of the window that pops up and paste in the following script:

setInterval(function() { document.querySelector('#primary button[aria-label="Action menu"]').click(); var things = document.evaluate('//span[contains(text(),"Watch later")]',document,null,XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,null); for (var i = 0; i < things.snapshotLength; i++) { things.snapshotItem(i).click(); } }, 5);



Now press ENTER and watch the magic happen...If you see videos stacking up that aren't being deleted, especially greyed-out [private/deleted/removed video] videos, click the 3-dot icon on each of those greyed out videos, and they should be eliminated along with the rest. If the process pauses, you might be able to help it out by moving the scroll bar on the video list to scroll toward the bottom of the list. If the scroll bar isn't helping, you may have to refresh the page and reload the script.  You can also experiment by adjusting the last number in the script. Higher numbers take longer, lower numbers are faster, but your effectiveness at each speed may vary.

Good luck, and may the force be with you!