Initial and early amendments of FAR 23.51 that requirement only applied "For airplanes of more than 6,000 pounds maximum weight"
Yes, which is why I referred to "applicable". Lighter planes are sometimes (for my experience testing them) even more susceptible to difficulty gliding safely from a Vx climb, so I consider the "more than 6000 pound rule" suitable to apply to less than 6000 pound airplanes also, as they don't have a prescriptive performance standard. I find that this characteristic of airplanes is poorly appreciated by many pilots (though very well appreciated for helicopters by helicopter pilots), so I'm more cautious when demonstrating design compliance during testing modified planes (floatplanes, in particular), and more likely to train it to unfamiliar pilots.