Question Projection of faces on the Y axis

Hi, I'm in Unity URP making a shader that generates a mask to block another shader. I want this mask to be generated by blocking faces based on the Y axis. Below is a diagram of what I have in mind:


The faces that form a right angle with the ground would not be affected unless there is something on top of them. I would like these occlusions to be reduced by distance. As the example a below