Seen this function in a car forum, non functional links are disabled. You can't click them and if you move the mouse over that link a popup shows the link address and the reason why the link isn't active.

Here you can see it

I don’t like this at all. Even if the link is broken, I still might want to visit the site and get information from the error page or by changing the URL or by browsing the site to find moved content for example. But on the forum you show, I can’t even copy and paste the URL since it is cut off. 

I also wonder how this is done technically. I doubt this is a live check. This would slow down the page loading significantly. If it is cached, then it could report URLs as broken, even if they came back online after it was checked.