You can enumerate and extract nav areas and its flags by observing this plugin by BHaType.
Here is my fork to 3D-visualize areas, nearest to player origin.
It's for l4d1 only. You can use above plugin to adapt to l4d2 if required.
More Area Types can be found here: https://developer.valvesoftware.com/...n_Meshes_(L4D)