Based on some quick research, modern browsers have a setting to override page settings for Visited and Unvisited links, so anything used on a site such as Groups.io is only a suggested default.
I changed the Visited / Unvisited colours in my browser (FF) and as far as I could see it made no difference whatsoever to the appearance of links within Groups.io.
As a rule this is not something that bothers me on Groups.io but I can see Jason's point. Another site I regularly use has links that change from blue to an unmistakable red once visited.
