To better understand what's up with the hevc-legacy checkbox being gone in the vp21 build 187 update, I did some quality analysis compared to vp20 whose numbers I already had handy. All these are from my laptop with Nvidia 3060 for gpu and either Intel IrisXe QSV decoding or Legacy-Hevc.
It's a morass of numbers but pretty obvious to "Vegas Pro-CSI" that the new Qsv is based on the old Legacy-Hevc. And that the old Qsv is evolving into the new Experimental Hevc. The metrics being much like decoder fingerprints. I'm so happy the quality of the old Legacy-hevc has not gone down the tubes! Next I've got some performance analysis that's been keeping me busy. Give me a few moments.