Currently, this is not the case - the player is not allowed to fly above ceilings or below floors, their Z position is clamped to the floor or ceiling of the sector they're in or, if they're out in the void, the sector they were last in. AFAICT it behaves identically to NOCLIP + FLY.The player's z-movement is completely unrestricted (i.e. he/she is allowed to fly above ceilings and below floors). This is the main thing that separates it from standard NOCLIP + FLY.
As some of the comments in the thread point out, a big use of this cheat would be to fly up out of your current room and see a view of the surrounding level from high up. The current behavior does not make that possible. I'm working on a very large map right now and noclip2 is only marginally more useful to me than regular noclip.
If this kind of feedback / suggestion(?) is better suited for the bug tracker I'm happy to create an issue there.
If anyone depends on the current behavior, maybe make a noclip3 that behaves as originally requested?