I’m experimenting a bit with the following method to find AeT (VT1/LT1) and compare the result with other methods. Thought others might be interested too:
Yes I was a big Maffetone follower (the king of low heart rate training) and was / am a low HR practitioner and I think HR drift is a very clear indicator (at least from Maffetone’s and my perspective). I could run for hours with basically zero drift if I was under AeT. I know in Maffetones case he felt high fat burn ratio was important as well, so limiting carbs before and during activities. Not sure how important that is but I was doing it as well. I could run for 2.5 hours without food (about 8 years ago… before kids)…
So yes my HR drift limit seems to align with my first deflection point using SMO2 / Humon. In my case that was around 130 bpm for a 183 max. Not sure now. My primary training is chopping with axes and using a chainsaw with my kids lately 
You may find it interesting to look at Stephen Seilers work. He has used looked at HR drift to see if the athlete has recovered from a prior effort. He has developed a metric %HRR/%6minpower which tends to be a constant over zone 1+2 intensities through rides of 1-4 hours. Finding the point where drift is between 3 and 5% likely needs to be in a well rested state for it to be a marker of LT1. I suspect giving enough time for recovery is somewhat key to advancing performance and is something I am only just starting to understand so whilst these metrics take a little time to work out they could prove useful.
HRR is heart rate reserve ie max HR less resting HR.
The metric you’re talking about is implemented in intervals.icu and that gives a very nice view of the decoupling trend over a longer workout. An example from a short LIT session yesterday with zero decoupling:
Oh interesting. Yes I see the topic here
What’s funny is Dr Maffetone talked about HR drift determining AeT 13-years ago (2010). Not sure it was this book exactly, but this is a good one. He also talked about the importance of staying under AeT way back then.
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Colour me interested. I sense another rabbit hole in my future 
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