Delete watched YouTube videos from Watch Later. Only tested on the old layout.

remove-youtube-watched.js

const findVideos = () => [...document.querySelectorAll('.pl-video')];

const watchedGreaterThanHalf = (video) => {
  const progressBar = video.querySelector('.resume-playback-progress-bar');
  return progressBar && parseInt(progressBar.style.width) > 50;
}

const removeButton = (video) => x.querySelector('.pl-video-edit-remove');

const click = (button) => button.click();

findVideos()
  .filter(watchedGreaterThanHalf)
  .map(removeButton)
  .forEach(click);