Forked from https://gist.github.com/Acendro/098f82e014a21ea18cad25af77d0bcff

youtube-playlist-cleanser.user.js

// ==UserScript==
// @name         Youtube Playlist Cleanser
// @version      2.1.0
// @description  Removes watched videos from playlist either by %watched or all
// @author       js6pak
// @include      http*://*.youtube.com/*
// @include      http*://youtube.com/*
// @run-at       document-idle
// @homepageURL  https://gist.github.com/js6pak/33bdefdefac09c387f55d08c5b9526fa
// @downloadURL  https://gist.github.com/js6pak/33bdefdefac09c387f55d08c5b9526fa/raw/youtube-playlist-cleanser.user.js
// ==/UserScript==

const config = {
  // Delete if watched more or equal to % of video
  threshold: 80,

  // Delay between delete requests (seems to need to be quite high for consistent results)
  delay: 600,
};

const app = document.querySelector("ytd-app");
if (!app) return;

const sleep = (timeout) => new Promise((res) => setTimeout(res, timeout));

function waitForElement(selector) {
  return new Promise((resolve) => {
    if (document.querySelector(selector)) {
      return resolve(document.querySelector(selector));
    }

    const observer = new MutationObserver(() => {
      if (document.querySelector(selector)) {
        resolve(document.querySelector(selector));
        observer.disconnect();
      }
    });

    observer.observe(app, {
      childList: true,
      subtree: true,
    });
  });
}

function createButtons(menu) {
  const cleanseButton = document.createElement("button");
  {
    cleanseButton.textContent = "Cleanse";

    cleanseButton.style.padding = "10px";
    cleanseButton.style.backgroundColor = "#181717";
    cleanseButton.style.color = "white";
    cleanseButton.style.textAlign = "center";
    cleanseButton.style.fontSize = "14px";
    cleanseButton.style.border = "0";
    cleanseButton.style.cursor = "pointer";
    cleanseButton.style.fontFamily = "Roboto, Arial, sans-serif";
    cleanseButton.style.borderRadius = "2px";
    cleanseButton.style.marginRight = "10px";

    cleanseButton.addEventListener("click", function () {
      cleanse();
    });
  }

  const deleteAllButton = document.createElement("button");
  {
    deleteAllButton.textContent = "Delete all";

    deleteAllButton.style.padding = "10px";
    deleteAllButton.style.backgroundColor = "#ff0000";
    deleteAllButton.style.color = "white";
    deleteAllButton.style.textAlign = "center";
    deleteAllButton.style.fontSize = "14px";
    deleteAllButton.style.border = "0";
    deleteAllButton.style.cursor = "pointer";
    deleteAllButton.style.fontFamily = "Roboto, Arial, sans-serif";
    deleteAllButton.style.marginRight = "10px";

    deleteAllButton.addEventListener("click", function () {
      cleanse(true);
    });
  }

  menu.prepend(cleanseButton, deleteAllButton);
}

function* getVideos() {
  const videos = document.querySelectorAll("ytd-playlist-video-renderer");

  for (const video of videos) {
    const title = video.querySelector("#video-title").innerText;
    const progress = video.querySelector("ytd-thumbnail-overlay-resume-playback-renderer")?.data.percentDurationWatched ?? 0;
    const menu = video.querySelector("ytd-menu-renderer");
    const menuButton = menu.querySelector("yt-icon-button#button");

    yield {
      container: video,
      title,
      progress,
      menu,
      menuButton,
    };
  }
}

async function deleteVideo(video) {
  video.menuButton.click();

  const popup = await waitForElement("ytd-menu-popup-renderer");

  Array.from(popup.querySelectorAll("ytd-menu-service-item-renderer"))
    .find((x) => x.icon === "DELETE")
    .click();

  await sleep(config.delay);
}

async function cleanse(deleteAll = false) {
  console.log("Cleansing...");

  let deletedCount = 0;

  for (const video of getVideos()) {
    console.log(` ${video.title} (${video.progress}%)`);

    if (deleteAll || video.progress >= config.threshold) {
      console.log("  Deleting...");

      await deleteVideo(video);

      deletedCount++;
    } else {
      console.log("  Skipping because its under threshold");
    }
  }

  console.log(`Done! Deleted ${deletedCount} videos`);
}

waitForElement("ytd-playlist-header-renderer ytd-menu-renderer").then((menu) => {
  createButtons(menu);
});

function createQuickDeleteButtons() {
  for (const video of getVideos()) {
    const quickDeleteButton = document.createElement("yt-icon-button");

    quickDeleteButton.className = "style-scope ytd-menu-renderer";
    quickDeleteButton.setAttribute("style-target", "button");
    quickDeleteButton.style.marginRight = "10px";

    video.menu.insertBefore(quickDeleteButton, video.menuButton);

    const deleteIcon = document.createElement("yt-icon");
    deleteIcon.className = "style-scope ytd-menu-renderer";
    deleteIcon.icon = "DELETE";
    deleteIcon.style.color = "#F44336";

    quickDeleteButton.querySelector("#button").appendChild(deleteIcon);

    quickDeleteButton.addEventListener("click", function () {
      console.log("Quick deleting " + video.title);
      deleteVideo(video);
    });
  }
}

waitForElement("ytd-playlist-video-renderer ytd-menu-renderer").then(() => {
  createQuickDeleteButtons();
});

This script quit working for me recently. I no longer had the Delete and Cleanse buttons appearing on my playlists.

I'm not a JavaScript programmer, but I took a stab at the code. I modified line 142 as follows:
original: waitForElement("ytd-playlist-header-renderer ytd-menu-renderer").then((menu) => {
changed to: waitForElement("ytd-item-section-renderer ytd-menu-renderer").then((menu) => {

Now the buttons appeared in the first item displayed, but they also appeared anywhere there was a list of YouTube videos, so I also changed the include scope comments at the top of the code, lines 6 and 7, as follows:
original:
// @include http*://*.youtube.com/*
// @include http*://youtube.com/*
changed to:
// @include http*://*.youtube.com/playlist*
// @include http*://youtube.com/playlist*

I'll leave it to someone more experienced in JavaScript than I am to do a proper fix if necessary.