forum-redirect.user.js

// ==UserScript==
// @name        Elite Dangerous: old forum URL redirector - frontier.co.uk
// @namespace   rimspace.net
// @match       *://forums.frontier.co.uk/showthread.php/*
// @grant       none
// @run-at      document-start
// @version     1.0
// @author      Daniel Pittman <daniel@rimspace.net>
// @description when you land on an old forum URL, instead redirect to the "modern" equivalent.
//              does not try and rewrite the URL before you hit the 404 page, though.
// ==/UserScript==
const rewriter = /showthread[.]php\/(\d+)-([^/]+)$/;
let match = null;
if ((match = rewriter.exec(document.location.href)) !== null) {
  document.location.href = `https://forums.frontier.co.uk/threads/${match[2]}.${match[1]}/`;
}