Q:

Level too difficult: nerfed

I remember reading that a particular level on Candy Crush received a flood of complaints, so King made it easier.  Did this happen, and which level was it? 

A:

Nerfing or buffing are also called Redesigning, and this happens all the time with many different levels.  So the answer for your question is yes, but not just one particular level. 
From Candy Crush Saga wikia

Every now and then, when too many people complain that a level is too
  difficult and many players reach the level and quit, the Candy Crush
  team will redesign the level to make it a little easier, or a lot
  easier in some cases. 

There are many levels have been redesigned (including both nerfed and buffed).  For the full list, you can have a look here.