Not "knowledge required to do well" on the y-axis?
That wouldn't change over time. The knowledge required to do well would be a constant.
I think the thrust of sayings like "the learning curve is a wall" is to indicate that there is little to no slope, which is to say you don't get the benefit of time spent learning easier parts step by step until you know a lot of the system and can handle the more difficult aspects of it. Instead it feels like you either know how it all works or nothing about how it works, and it's hard to figure out how to start and get a foothold, so to speak.
Thus your expression "an absolute wall" would mean that you immediately grasp everything there is to know.
But since the meaning since has unintentionally shifted and the original meaning is universally ignored, we all understand you.