THD, no, it’s worthless. The individual orders as a graph, not a single number, yes, that’s very useful. Granted it’s probably not that useful to you, but it’s invaluable to engineers designing a headphone.

Drivers are dumb objects, and will reproduce anything you give it, they can’t differentiate between frequencies. You can design a driver to reproduce significantly above 20kHz on purpose, but that isn’t done very often. Being able to produce >20kHz is just how the driver and pad and enclosure design will often turn out. Just about any modern headphone will be capable of producing at least slightly >20kHz at lower volume than the rest of the range.

You don’t stop ultrasonic frequencies from being reproduced with the headphones, you stop them in the mastering step of creating music by cutting them off, by rendering the song at 44.1kHz sampling rate, or by setting the audio output to your DAC to 44.1kHz.