EDIT (5:37 PM CDT): The studio's responded via Twitter:
EDIT #2 (6:28 PM CDT): It was a rating change:
We do not know why Infinite Minigolf seems to have disappeared from the eShop in North America. We are investigating!
Infinite Minigolf had a change in ESRB rating and was removed temporarily from the eShop. We are patching the game. It will be back ASAP.