Greasy Fork镜像

it is not working at all, half of these already watched.

This has already been discussed. It's how YouTube serves the related videos (videos on the right side of the watch page).
There's nothing I can do as those don't have any indicator that they are watched.
Read more here.

I also have a note on the install page.
I'm sorry but there's nothing I can do to fix that with css alone.

Like I said, the issue you reported is not from this css script, Youtube had broken it and now it seems to finally be fixed after a year.
I'd appreciate if you update your review rating. Thank you.