I can confirm that the feature doesn't seem to be working.
Two sources I found gave this as a definition for the feature: It should flag "hyperlinks that have broken or missing URL links"
I'd file a bug report here:
That's what I thought. Bug report submitted.
I think the option "Inaccessible URL Links" was introduced in InDesign CS5, back in 2010.
At that time, there was a feature called 'Place from Buzzword...' and the option 'Inaccessible URL Links' used to check if a linked Buzzword document was inaccessible and used to flag that error (if the option was checked).
I don't think it ever worked/checked for website links even then.
The 'Place from Buzzword' was removed in InDesign CS6 but this option has remained in the Preflight dialog.
I do remember it. The "Place from Buzzword" feature was little used, and eventually removed. It would be less confusing if that option was removed from Preflight dialog.
So why on earth is the checkbox still there? Surely it can be removed on the next patch/update... or turned into a working feature.
Here are three reasons: (1) At the time the "Place from Buzzword" feature was removed, no one involved remembered that the Preflight was checking for inaccessible URLs. Remember that InDesign is a very complex application with literally thousands of features. (2) Almost no one was using the feature. (3) Simply asking in this forum doesn't mean it will be added to the next patch/update. The people who make those decisions don't have the time to read every forum post.
To ensure that it gets noticed, make a request here: