Vim Keys for Reddit is a Safari extension that adds a number of simple keyboard shortcuts you can use when browsing Old Reddit. It was created because the Reddit Enhancement Suite dropped support for newer versions of Safari.
Vim Keys for Reddit is open-sourced here.
To install, download the latest version, extract it, and drag the app to your Applications folder. Then, open your safari preferences, navigate to the "Extensions" section and make sure it's enabled.
These older releases aren't available on GitHub. Here they are.