Some JS that will go through your Watch Later playlist on Youtube, and remove items sequentially

yt-remove-watch-later.js

/**
 * Works on your watch later playlist `/playlist?list=WL`
 * Update NUMBER_OF_LATEST_VIDEOS_TO_KEEP if you want to keep more/less than the  20 most recent videos
 */

const NUMBER_OF_LATEST_VIDEOS_TO_KEEP = 20;

async function doThing(vidArray) {
  for (const vid of vidArray) {
    vid.click();
    await Promise.allSettled([
      poll(() => {
        const removeBtn = document.querySelector(
          "ytd-popup-container ytd-menu-service-item-renderer:nth-child(3)"
        );
        if (!removeBtn) {
          return null;
        }
        console.log("removeBtn", removeBtn);
        removeBtn.click();
      }),
      wait(1200),
    ]);
  }
}

function wait(milliseconds) {
  return new Promise((resolve) => {
    setTimeout(resolve, milliseconds);
  });
}

function poll(fn, timeout, interval) {
  var endTime = Number(new Date()) + (timeout || 1200);
  interval = interval || 600;

  return new Promise(function (resolve, reject) {
    (function p() {
      // If the condition is met, we're done!
      if (fn()) {
        resolve();
      }
      // If the condition isn't met but the timeout hasn't elapsed, go again
      else if (Number(new Date()) < endTime) {
        setTimeout(p, interval);
      }
      // Didn't match and too much time, reject!
      else {
        reject("timed out for " + fn + ": " + arguments);
      }
    })();
  });
}

const vidBtns = Array.from(
  document.querySelectorAll(
    `#contents ytd-playlist-video-renderer:nth-child(n+${NUMBER_OF_LATEST_VIDEOS_TO_KEEP}) yt-icon-button button`
  )
);

doThing(vidBtns);