Doesn’t work (at least for me in Chrome with Tampermonkey) due to Hacker News’s Content-Security-Policy header. > Refused to load the image ' ' because it violates the following Content Security Policy directive: "img-src data: 'self' ". Overriding this would probably require an extension with a Service Worker that edits the CSP header.
This modification works for me: https://greasyfork.org/en/scripts/443687-hn-favicons . It uses GM.addElement to bypass CSP restrictions.
Unfortunately, it still doesn't help in Tamper Monkey in Safari.