pprune fan#1
You have pointed out an area that only adds to the relative lack of information on the HV curve. And that lack of knowledge breeds misinformation in many areas.
If you get a copy of the Advisory Circular 27-1, it explains quite nicely how the HV curve is determined for certification purposes. Unfortunately, that information is not presented anywhere in the flight manuals, and until my books were published (unabashed advert for same), it wasn't explained anywhere.
Above the 'knee', the test is done from power for level flight, with a one-second intervention between throttle chop and collective intervention. Below the 'knee', the power is takeoff power and no intervention is used between failure and pilot action (the logic is that you had better have your hand on the collective on takeoff...)
So, it is not reasonable to expect the HV curve to apply to a landing profile where you are decelerating and at a lower power setting than level flight.
As far as I know, there is also no operating rule (at least in the FAA world) that requires you (in the Part 27 or Part 29 <9 pax world) to stay outside the HV curve at any time - can't remember the CAA rules, but you might check for them.
Terrain underneath will make a mockery of the HV curve even if you're outside of it in mosts cases anyway.
But your message about 'where is this written down?' is a valid one.
wish it were otherwise.