Twitch_MuteAds.user.js

// ==UserScript==
// @name    Twitch - Mute ads
// @include https://www.twitch.tv/*
// @version 1.0
// @license MIT
// @grant   none
// ==/UserScript==
(() => {
	window.setInterval(() => {
		let ads = document.querySelector('div[data-test-selector="sad-overlay"]');
		let muted = document.querySelector('video').muted;
		if(ads) {
			if(muted === false) {
				document.querySelector('video').muted = true;
			}
		} else {
			if(muted === true) {
				document.querySelector('video').muted = false;
			}
		}
	}, 1000);
})();