I don't understand. For you to see the message, you have to click on the link. Your clicking on the link must mean that the link is active, since it is getting clicks. So why is the link being deactivated for being inactive?
But that doesn’t make any sense. A link might be printed in a book. Nobody accessed it, yet they might at some point. Such a service is quite good for printing in books, instead of QR-codes.
If I had to guess it is possibly something to do with fighting crawlers/bots/etc triggering the detection? And running some kind of more advanced logic to try ensure it's really being used. Light captcha style.
Apparently they measured it once by running a map-reduce or equivalent.
I don’t see why they couldn’t measure it again. Maybe they don’t want it to be gamed, but why?