Flaps, like the gear and the whole aircraft have a Speed Limit (CAS) at lower altitudes, and a Mach Number limit at higher altitudes. Many manufacturers (example Douglas
) published Mach Number limits for Flap operation as well as CAS limits. Boeing's solution to not having to establish and prove the Flap Mach No. limit was to simply apply an altitude limit which is below the limiting CAS/Mach crossover.
A simple solution, but yes, it does prevent you from deliberately exceeding 20,000 feet in the event of Flaps stuck in the extended position and diversion to alternate - you would be operating outside the certification envelope.
You might notice that the gear does have a Mach as well as a CAS limit, for the very good reason that gear may be used in an emergency descent from high altitude in the event of speedbrake failure.
Regards,
Old Smokey