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You'll need to perform some level of 3D mapping in order to detect the stairs. What you're actually trying to detect is an absence of floor as opposed to the presence of an obstacle. For this task depthimage_to_laserscan is not longer appropriate because it removes any information about the floor.

What I'd recommend is to keep using the laserscan method for localisation but adding an additional process that adds a map layer using the full pointcloud of the RGB-D sensor. This process detects the presence of the floor by checking their are points in that plane and creates a map layer of known safe floor areas, the cliff hazards are then the inverse of this layer.