Challenge Caches have gone belly up. This was GC management's 2nd 'virtual' game response to the ever-present 'bring back virtuals' request.
Their first new 'virtual' game to appease the masses was/is Waymarking, more like Locationless caches, not really like old-fashioned Virtuals, so the beseeching continued.
The now defunct Challenge Caches were introduced just a few months ago. They are more like a photo location game - 'go somewhere, do something', post a photo. Almost nothing like old-fashioned Virtuals where you needed to visit the location and prove that you found the cache by answering questions that could be answered by physically visiting the location.
Maybe geocaching.com management might re-consider bringing back real circa 2002 Virtuals. But to control them, so they don't get out of hand - charge $5 per virtual hide. People will probably be more selective about where they put a Virtual cache if there's a small monetary investment involved. The idea comes from the Munzee site. Virtual munzees got out of hand and were abused but are more controlled now that Munzee management put a price tag on them.