In an email in Nov. 2015 it said:
"Last April, we paused accepting submissions for new challenge caches to collect data and reevaluate the impact they have on the game."
Then (premium?) Users could take a survey on challenge caches.
Last April Groundspeak said, "[...] effective April 21, 2015, we're pausing accepting any submissions for new challenge caches for one year.[...]"
They did not announce, that they would not accept new challenge caches in the future, nor did they say that they are accepting new challenges after April 22, 2016.
We have to wait and see what will happen to the Challenge Caches.