ISM would use lactate to come up with both Power zones and HR zones. Personally I use power and only look at HR if something isn’t going well. Some do the opposite.
FWIW I’ve received LTHR updates from TrainingPeaks since 2016, and on my Garmin 530 for 3+ years. The Garmin 530 gives a lot of estimates, averaging about once a week. Except when I’m fully detrained, the LTHR estimates from my Garmin are usually within 2bpm and are consistent with what I see after 10+ minutes riding at ftp (when doing threshold intervals). I would not rely on % LTHR as detraining will cause my Garmin to show a 4-6bpm drop which is gone with 3-4 weeks.