YouTube_always-mobile.user.js

// ==UserScript==
// @name          YouTube always-mobile
// @namespace     https://github.com/UCyborg
// @description   Always redirects desktop YouTube to mobile version
// @version       1.0.1
// @author        UCyborg
// @downloadURL   https://gist.github.com/UCyborg/100935b7572423a56e28fb0e60596c92/raw/YouTube_always-mobile.user.js
// @include       /https:\/\/www\.youtube\.com\/(?!embed\/.*$)/
// @run-at        document-start
// @grant         none
// ==/UserScript==

'use strict';

(function() {
  let url = new URL(window.location);
  url.host = 'm.youtube.com';
  url.searchParams.set('app', 'm');
  url.searchParams.set('persist_app', '1');
  window.location.replace(url);
})();