Maybe not per the manufacturers, but it definitely checks out. I’ve run plenty of logging on my previous NA engines (two Jeep 3.6s, a Subaru 2.4, and a Ram 6.4) on 85 when I was living at 9k feet and no knock or timing pull at all when at altitude, including towing in the heat.
You do of course need to be careful not to step down octane if you’re going to transition from high altitude to low altitude..
Yep, did many a trip from Kansas City to Denver/mountains above. Going west, at Limon 85 coming back 87 and never forget... It's interesting, but in MT, where I live now, no 85 so no question in my 2500 Hemi but I don't put 85 in even if I have the opportunity... I remember going west with my parents pulling a pop up as a kid year after year... Dad had a 64 Dodge station wagon with a 440 wedge engine and hydraulic clutch. It was an odd one they were having a hard time selling so he got a deal... It was a beast of a car but, he would have his mechanic do a high altitude tune on it every year when we headed west... Since he was a Jr. High School principal, we would take off the day after he closed his school and stay out for as long as 60 days, usually 30 to 45 days...
I was never sure what that high altitude tune meant or did. But, dad did tell me about the octane difference and how the altitude made an 85 act effectively like an 87 at low altitude, but back in those days, it was ALL carburetors all the time, non of that fancy smancy fuel injection stuff...