A bit off topic, but within the spirit of responding to points raised within the thread.............
Equivalent True Airspeed (ETAS) = TAS X Cosine of Drift Angle
It's a Navigational function, not an aerodynamic one, I think that the poster must have meant something else
The 20000 ft Pressure Height limit is so that the manufacturer need not certify the Mach Number limit for Flap operation, as others have indicated.
(And as addressed in numerous other threads).
Regards,
Old Smokey