I encourage you to get everything out of your Watch Later list. Even if you aren't trying to only get videos from way back when recommended only, there are good reasons:
1. You have hundreds of even thousands of videos in there. It's like hoarding. Just let it go. If you are meant to see them, they will eventually be recommended to you again.
2. Those videos don't suit your current interests and you know it.
3. They are likely continuing to recommend you things based on your Watch Later, which means your recommended is polluted with garbage.
4. If you're like me, you used to watch people who turned out to be awful, and their stuff is in your Watch Later, long forgotten. Nobody wants to be jump scared with a DSMP recommendation.
If deleting 3k videos by hand doesn't sound like a fun time, you may want to consider the possibility of deleting that YouTube account. If you make another one, make sure your cache is cleared, and you don't have linked Google accounts, or you'll still get things in your recommended that indicates YouTube remembers your previous account.
If your goal is to get rid of AI only from your recommended, then add this to your searches on both YouTube and Google:
before:2022-08-22
However, if you want to also cull Mr Beast style retention editing from your recommended, do this:
before:2017-01-01
Your next course of action after you've cleared all your history and made YouTube forget you is to go look up things using this method, make a playlist of videos from before the unwanted date, and then remember to turn off watch history if you're trying out a new channel or watching something that you likely won't want recommended to you. Take a few days to a week of just letting these videos play while you aren't there so that YouTube starts recommending the correct content to you. It will still try to push new videos, but it'll eventually be 90-95% older videos after awhile.
At first, YouTube will be confused about what you want, and recommend, for example, anime videos from every year. If you indiscriminately watch videos that are older, it will wonder what you're planning. But then, it will finally give only videos that are old. Always remember to not watch newer things (unless it's from a trusted channel that you know doesn't meet the criteria for content you want to avoid) with watch history on.
This works for all websites. You can cull all AI from every search if you do the first search month I recommended. So, until we get a website that isn't garbage, you can still use Pinterest through Google:
On YouTube, you can also cull names of creators, and words in titles. Not adding the year this time.
Minecraft -dsmp -dream -george -willbur
You can cull websites from searches.
Poses for art -pinterest -reddit
If you know how to use tamper monkey, you can have a lot of stuff done for you in regards to certain specific searches. I'd explain how, but I need to learn more before I talk.