contentScript.js

function deleteVideoFromWatchLater() {
    video = document.getElementsByTagName('ytd-playlist-video-renderer')[0];
    video.querySelector('#primary button[aria-label="Action menu"]').click();
    var things = document.evaluate(
        '//span[contains(text(),"Remove from")]',
        document,
        null,
        XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,
        null
    );
    for (var i = 0; i < things.snapshotLength; i++) {
        things.snapshotItem(i).click();
    }
}

async function deleteWatchLater() {
  // Fiddle with these if you'd like
  let batchSize = 200; // Number to delete at once before waiting
  let waitBetweenBatchesInMilliseconds = 1000 * 60 * 5; // 5 minutes
  let waitBetweenDeletionsInMilliseconds = 500; // Half a second

  let totalWaitTime = ((5000 / batchSize) * (waitBetweenBatchesInMilliseconds / 1000 / 60)) + (5000 * (waitBetweenDeletionsInMilliseconds / 1000 / 60))
  console.log(`Deletion will take around ${totalWaitTime.toFixed(0)} minutes to run if the playlist is full.`);

  let count = 0;
  while (true) {
    await new Promise(resolve => setTimeout(resolve, waitBetweenDeletionsInMilliseconds));
    deleteVideoFromWatchLater();
    count++;

    if (count % batchSize === 0 && count !== 0) {
      console.log('Waiting for 5 minutes...');
      await new Promise(resolve => setTimeout(waitBetweenBatchesInMilliseconds));
    }
  }
}