I am not. If anything, I was explaining why there is no need for that specific measurement. If 90 kHz bandwidth THD+N is fine, there is no reason to suspect significant high frequency distortion. However, according to DonH56, THD is conventionally measured using the first 10 harmonics which requires a 200 kHz bandwidth for a 20 kHz fundamental.
Most DACs have steep filters above 20 kHz so THD at 20 kHz is pretty meaningless. Bandwidth will always reduce THD, of course, which is one reason it was sometimes just a 1 kHz measurement in the past. Bandwidth of the device must be taken into account when looking at THD, but an argument can be made that if the bandwidth limits the number of harmonics that contribute to the THD, then that also affects what we hear and thus is a reasonable measurement. It can mask the measured distortion, however, and is one reason I like to see 19+20 kHz IMD testing, so you can see the 1 kHz product.
Class D amplifiers are a case (to me anyway) for wideband spectral analysis, not necessarily for distortion, but to check for oscillator/switching feedthrough and intermodulation products in the audio band. I recently read about an amp blowing tweeters the instant it was turned on, and the root cause was a DIY modification that shorted the output filter inductor because the owner had read how inductors limit high-frequency response. They do, very intentionally in this case, natch. A little knowledge is a dangerous thing...
BTW, to be clear my day job after graduation was never audio, so there are likely better experts to quote regarding all things audio. The ten harmonics criteria is from my audio days as an IHF consultant (there was a series of volumes to read and tests to pass at the time) and was what manufacturers and reviewers used at the time best I recall. The IHF and some other standards bodies (I am an IEEE member so use them most of the time) specified at least 10 harmonics, but of course if the amplifier does not have the bandwidth for 10, the measurement will reflect that. In practice, second and third usually dominate the distortion, with very little contribution past the 5th or 7th harmonic for real-world designs. Past that they are often buried in the noise floor.
FWIWFM/IME/IMO/etc. - Don