“This proof of concept looks somewhat like a reCAPTCHA challenge, and styles visited links to look like black squares.”
Browsers locked down the :visited selector so scripts can’t read which links you clicked. So the page asks you to click them instead. You think you are proving you’re human. You are handing over your browsing history one square at a time. Years of CAPTCHAs trained everyone to click whatever boxes a page tells them to.