2023-12-14. To restore animated thumbnail previews probably requires inline previews to be disabled in your Youtube settings, but if you haven't done that you won't be looking for this userscript anyway.
// ==UserScript==
// @name Restore animated thumbnail previews - youtube.com
// @namespace Violentmonkey Scripts seekhare
// @match http*://www.youtube.com/*
// @run-at document-start
// @grant none
// @version 1.0
// @license MIT
// @author seekhare
// @description 2023-12-14. To restore animated thumbnail previews probably requires inline previews to be disabled in your Youtube settings, but if you haven't done that you won't be looking for this userscript anyway.
// ==/UserScript==
Object.defineProperties(Object.prototype,{isPreviewDisabled:{get:function(){return false}, set:function(){}}})