Spoiler warning: If you do not know yet, but want to read the Terry Pratchett novel Going Postal, please stop reading here. Spoilers below and in the links!
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The page XClacksOverhead.org lists public-facing websites broadcasting the X-Clacks-Overhead header aka Known websites carrying the signal.
I learned about this tribute to Terry Pratchett from a recent Golem.de article [German]: GNU Terry Pratchett: Eine Hommage für Eingeweihte an den Scheibenwelt-Erfinder
Citing Wikipedia:
Users of the social news site Reddit organised a tribute by which an HTTP header, "X-Clacks-Overhead: GNU Terry Pratchett", was added to web sites' responses, a reference to the Discworld novel Going Postal, in which "the clacks" (a semaphore system, used as Discworld's equivalent to a telegraph) are programmed to repeat the name of its creator's deceased son; the sentiment in the novel is that no one is ever forgotten as long as their name is still spoken.
A June 2015 web server survey reported that approximately 84,000 websites had been configured with the header.
Supposedly, the invisible header is still there.
The markdown link was done as a way for individuals to join in on it.
It uses the standard link markdown of [x](y). On reddit it displayed as a single . showing with the GNUTerrypratchett beside it.
Now, the official app nor the site don't recognize it either, despite it having worked for years on every app and the site. Hell, it's not even working in the barely functional 3rd party apps now.
I don't have any screen shots of it working to show what it looked like and why it was cool.
Thank you for the description, this sounds really like it was a cool, nifty feature :)