This is now discontinued. It still sort of works, but the method it uses could stop working any day now. I made CustomTube, a Firefox extension that does what this script did, and so much more.
Install here: https://addons.mozilla.org/en-CA/firefox/addon/customtube/
Github: https://github.com/lightbeam24/customtube
A CSS stylesheet that removes some of the changes to the YouTube watch page that were implemented on May 9, 2022. It currently uses a temporary method to fully restore the old layout, but will continue to partially work even after this method is removed because of the other fixes included.
It is available as a Firefox extension.
For non-Firefox users (or Firefox users who prefer), it is also available as a userstyle (userstyles.world), and a userscript (greasyfork.org).