Excellent question. The key difference between HR, on the one hand, and pace and power, on the other, is that HR is a physiological measure, whereas pace and power are performance measures. This difference becomes important when considering questions like the one you’ve asked. The 80/20 intensity zones are anchored to functional lactate threshold values for pace and power, which are indeed defined as the fastest pace/highest power an athlete can sustain for one hour. So, if your preferred intensity metric is either pace or power, it’s fine to use a one-hour time trial to determine this value.
With HR, though, not so much. If you do a one-hour time trial, you’ll find that your HR climbs throughout it. Sure, you can take your average and use it as an approximation of your LTHR, but how accurate is that, really? A physiological threshold is a pinpoint, not an average. So, if you’re going to train by HR, I’d rather see you determine your FTP, rFTP, and/or LTP and then find the HR that aligns with this value through separate testing.