Awesome! I do this with my YouTube Watch Later playlist and it really works. I'll get a couple hundred videos I "definitely want to watch, but not now" and my script will clear them out after X time. Never once have I missed something it's deleted. I don't even know _what_ it's deleted, because if it stood out enough to remember the name and search it up again I'll probably just watch it. Very few things do.
Do you mind sharing your Watch Later script?
It sorts by `Date Added (newest)` and truncates the list to the 150 most recent videos. It also removes anything I've watched more than ~80% of. (Because the built-in button removes videos if you've watched _any_ percent, incl long ones you haven't finished yet)
Script here: https://pastebin.com/Sfh6a0w1