I'm still having a hard time understanding what exactly this represents and how to properly interpret it.
I don't know any professional photographers who use 2D histograms for editing, or whether you can actually plot them in Lightroom or Photoshop. However for scientific image processing it seems to be more useful.
OpenCV also has a tutorial: https://docs.opencv.org/3.3.1/dd/d0d/tutorial_py_2d_histogra...
But which of the 'coming chapters' it's referencing I have no idea.
Why not? Couldn't you just create a 3D image?
BTW there are tools for visualizing and "touring" these kinds of slices in arbitrarily high dimensions: http://www.ggobi.org/demos/tour.html