this script allows the deleting of all videos from a youtube playlist, like the 'watch later' playlist
Open your playlist on youtube and then scroll down all the way to the last video, and then scroll up to the first video of the playlist.
Now click right click -> Inspect element -> click on the 'console' section that appears on the developer tools -> paste the code from the script.txt on that console to run (the console will have a > indicator at the bottom showcasing where to paste the code)
Now just wait for all the videos to get deleted automatically!