Fix old reddit URLs – Fixes problems with displaying old.reddit URLs. Examples of breakage include links with underscores, double quotes, or trailing closing braces. If you also want to enforce old.reddit via another userscript, see: https://gist.github.com/Rainyan/fbb1ff337eac3e6c604d9a6eee7badda

fix-old-reddit-links.user.js

// ==UserScript==
// @name           Fix old reddit URLs
// @description    Fixes problems with displaying old.reddit URLs. Examples of breakage include links with underscores, double quotes, or trailing closing braces.
// @version        1.4.1
// @namespace      redditFixOldLinks
// @updateURL      https://gist.githubusercontent.com/Rainyan/0fdf3d449404fb786ea1a29589608ce5/raw/
// @include        https://old.reddit.com/*
// @run-at         document-idle
// ==/UserScript==

// Optional: If you want to enforce old.reddit via userscript, see also:
// https://gist.github.com/Rainyan/fbb1ff337eac3e6c604d9a6eee7badda


// Will replace these patterns in hyperlink hrefs.
const URL_REPLACEMENTS = {
    "%5C\"": "%22",
    "%5C_": "%5F",
};

// Will replace these patterns in hyperlink titles.
const TEXT_REPLACEMENTS = {
    "\\\"": "\"",
    "\\_": "_",
};

const NUM_LINKS = document.links.length;

for (let i = 0; i < NUM_LINKS; ++i) {
    let hyperlink_title_equals_url = false;
    try {
        hyperlink_title_equals_url = (decodeURI(document.links[i].text) === decodeURI(document.links[i].href));
    } catch {
        // If it failed, we have a malformed URI, or most likely link title is not an URI. Do nothing.
    }

    for (const key in URL_REPLACEMENTS) {
        document.links[i].href = document.links[i].href.replaceAll(key, URL_REPLACEMENTS[key]);
    }

    // If hyperlink title is the same as the URL, also clean up formatting of the title.
    if (hyperlink_title_equals_url) {
        for (const key in TEXT_REPLACEMENTS) {
            document.links[i].text = document.links[i].text.replaceAll(key, TEXT_REPLACEMENTS[key]);
        }
        document.links[i].text = decodeURI(document.links[i].text);
    }
    // Special cases
    else {
        // This can fail on non-URI content, so we wrap it to an empty try-catch,
        // because there's no need to do anything if decode failed.
        try {
            // For some reason, Reddit can eat the trailing ")" character from an URL,
            // so manually checking for that. This can occur for Wikipedia disambiguation page links, etc.
            //
            // TODO: we will still fail for links that have text different from the href.
            // Example of such content: [fighting windmills](https://en.wikipedia.org/wiki/Tilting_at_windmills_(disambiguation)),
            // where old.reddit will end the URL on first ), and render the closing ) as text.
            // Should rework this to instead parse the immediately following text to detect the failed closing brace case.
            if (decodeURI(document.links[i].href) + ")" === decodeURI(document.links[i].text)) {
                document.links[i].href += ")";
                document.links[i].text = decodeURI(document.links[i].text);
            }
        } catch { }
    }
}

Awesome script! You should change the file extension to .user.js so Tampermonkey recognizes it as a userscript and offers to install it when accessing the raw link.

Thanks! I've updated the file extension.