FWIW, it’d be pretty trivial to add slope angle as the edge weight to this MeshPaths GHPython component I once wrote (i.e. add another weightMode to the two functions that generate the graph), which implements networkx for the graph computation. You can download the 1.5 version here and unzip it to:
C:\Users\USERNAME\AppData\Roaming\McNeel\Rhinoceros\7.0\scripts
Edit: Here’s a quick implementation of what I was suggesting above. I also added a Smoothen input, which iteratively averages the path vertices and projects them back onto the mesh in Z:
220531_LeastSteepMeshPath_00.gh (312.8 KB)