One of the reasons for this is that for a NA (normally aspirated) engine, i.e. not turbo charged or super charged, the lower atmospheric pressure results in a lower compressed pressure (versus sea level) as the piston rises toward the top, which means a lesser chance of pre-mature detonation (knock) due to pressure, which is what higher octane prevents. So therefore you don't need as high octane at higher altitudes.

Unfortunately that isn't true with turbo or super charged engines, which are more and more popular. I really miss the 93 (and even 95) octane fuel I could get in Boston. I use a octane booster in my car here in Colorado to help offset the lower octane pump gas, and allow my turbo charged engine to make more power (by not having to pull timing to accommodate the lower octane gas).

85 is functional at elevation, but definitely still not preferable at elevation.

Works best with carbureted engines, which are obviously few and far between these days. You shouldn't put 85 octane in a modern fuel injected engine period.