Good ol' Reddit v2

good_old_reddit_v2.js

// ==UserScript==
// @name        Good o'l Reddit
// @description	Makes sure you're using Good o'l Reddit. (C) TheNH813 2018, additions by Chana G. License WTFPLV2
// @version		1.0
// @match       *://*.reddit.com/*
// @run-at      document-start
// @grant       none
// ==/UserScript==

// old.reddit.com doesn't work on media
const pageIsMedia = window.location.pathname === "/media";

if ( window.location.host !== "old.reddit.com" && !pageIsMedia) {
    var oldReddit = window.location.protocol + "//" + "old.reddit.com" + window.location.pathname + window.location.search + window.location.hash;
    /* Automatically translated posts have URLs that contain something like an `/es` at the end which makes old reddit 404.
     * e.g. https://old.reddit.com/r/Bass/comments/143cqt6/amplificador_de_bajo_para_tocar/es/
     * removing the latter part fixes it.
     */
    if(oldReddit.includes("comments/")){
      const [formerHalfOfUrl, latterHalfOfUrl] = oldReddit.split("comments/");
      const postId = latterHalfOfUrl.split("/")[0]; // we only need the post ID. The post slug is automatically replaced anyways
      oldReddit = formerHalfOfUrl + "/comments/" + postId;
    }
    window.location.replace(oldReddit);
}