Aldo Great post! I too 100% believe the cause of eye strain in Apple devices is purely software-induced (at least for me, as I am not affected by PWM). And I also believe the root cause can likely be toggled, on and off.
I too ran Sierra (up to 10.12.6) originally on these Macs with absolutely no strain:
- 2013 Mac Pro (connected to a 2006 23" Apple Cinema display)
But I was successful in going past Sierra. That is, back in 2019 I successfully upgraded all four of those Macs directly to Mojave 10.14.6 (bypassing High Sierra), and was able to use each one with absolutely no eye strain... for at least some time; where last August, both the 2013 Mac Pro and 2014 MacBook Pro - with all system and app updates completely disabled - suddenly went "bad", within the span of two weeks, producing the same level of strain that I get from all modern Apple devices.
On these two Macs, I have tried everything I can think of to get a strain-free experience again; where an earlier post here goes into detail of a few of the things I have tried. But worth noting, going back to even the very first version of Sierra (10.12.0, where the first macOS versions are not always easy to track down), does nothing to help with the strain.
On both the 2013 13" and 15" MacBook Pro's, I installed Catalina at one point (back when I was still comfortably using Mojave), as a trial in testing if I should upgrade the other two Macs past Mojave. No matter what tweaks I make in Catalina (even down to the kext's), I always get strain with these older Macs. And unfortunately for these two 2013 Macs, reverting them back to earlier macOS versions (including Sierra) does nothing to help make them strain-free again, once having had Catalina on them. (Not sure if the issue has to do with a newer EFI boot loader, installed by Catalina, or what.)
However as I have tried going back as far as Mavericks 10.9.0 - which is the earliest OS these 2013 devices can support - and I still receive the same level of strain, I believe the strain does not start with High Sierra, and has instead been present in macOS/OSX for many years. Fortunately for you, whatever toggles the strain in the OS seems to at least be disabled for you in your Sierra installations.
I've been trying endlessly, carving out several hours a day, attempting to get these four Macs back to a strain-free state. If I can ever figure out the trick, it may help in diagnosing what is inducing the strain in macOS in the first place… as long ago I ruled out dithering as the cause of my strain, by disabling it for Intel GPU's through the AppleIntel kext's; so I firmly believe there is something else (i.e. not dithering) at play in Macs causing the strain.
Of note. On this forum, I have seen a couple of posts throughout the years where others have had 2013/2014/2015 "good Macs go bad", and were thankfully able to get them good again. So if anyone of you are reading this, I would be HUGELY GRATEFUL for advice on what worked for you.