Meaningless in what respect? In terms of using external control surfaces? More a function of air density than actual Mach number. The higher you go the more irrelevant Mach is.
The NF-104 had rocket thrusters because it went high enough the rudder and ailerons ceased to work.
https://youtu.be/TwpJ7fIDmCI
It was used when you went over 75,000 feet IIRC. Without them it was kind of like running out of speed in a rolling uphill scissors only more so. You could move the controls but the aircraft was basically in ballistic flight because the density of the air flowing over the control surfaces was inadequate to provide any meaningful control.
The RCS starts about 45 seconds in to the video.