// ==UserScript==
// @name        old.reddit redirect
// @description	Makes sure you're using Good o'l Reddit
// @license     WTFPLV2
// @version     1.0
// @author      TheNH813 2018, edited by me
// @match       https://www.reddit.com/r/*
// @match       https://www.reddit.com/user/*
// @run-at      document-start
// @grant       none
// ==/UserScript==

var oldReddit  = window.location.protocol + "//" + "old.reddit.com" + window.location.pathname + window.location.search + window.location.hash;
window.location.replace (oldReddit);

Edited so it still allows image preview, because old.reddit’ one is broken.