There are all sorts of far out problems that we can not be prepared for (that would be one hefty QRH), but I have never heard of the 20000ī limit to create any hazard. Surely we could one day have to do something that is prohibited, or at least not recommended to safe our bacon, and if I would have to top a 21000 foot mountain with flaps 1 just to get to an usable airstrip, I would send Boeing the test results.
MB