Cliff's can't be marked for destruction with a normal shift-click of a blueprint or copy-paste. Deconstruction planner still works (setting to disable that, too).
Clears any colliding ghosts.
Only works if the entity that marked the deconstruction was a player.
There's some issues with cliff collision detection and clearing ghosts. I'd have to do some extra math to get the collision to work right, which may not be easy. (there are references I can work from)