More of a curious question...

Has anyone figured out how to make air units unable to fly to a different cliff elevation?

I.e. Air units can fly on low ground but can't climb cliffs (like a ground unit).. but still act like an air unit?

Cause I remember that there're No Fly zones, but they aren't regions and stuff.... so I'm not too sure....

Air units with ground mover so they can't walk over the cliffs?
Collision still has to be the air unit collision for your purpose...

Ground units with the 3D model edited so it's several feet off the ground?

You can change the unit's height in the editor.

You can write a trigger to prevent a unit from moving somewhere with a certain ground elevation. Though there's almost definitely an easier, data-based way. Using a flying ground unit won't work, as it won't fly over depressions and water.

Can't you just use the "No Fly Zone" pathing feature?

EDIT:
Or pathing in general.