It turned out that what I downloaded was a full installer, and didn't need to install over a previous version.
I don't now if that Apple doc is deliberately misleading or just incredibly
poorly written, your experience with it being an excellent example.
My archived Yosemite, El Capitan, and Sierra installers appear to be the same dmgs to which the doc links, and they launch as expected in my Big Sur installation.
My archived High Sierra, Mojave, and Catalina installers do not launch, popping this
I was already aware of the hack you mentioned, but I doubt that it will work in this instance, because not a one of those installers is old enough to actually be out of date. I suspect it's a function of my trying launch them in Big Sur, but I"m at a total loss as to why that might be the case.
The App Store links to the High Sierra and Mojave installers return this
, while the Catalina link allows me to d/l the installer which, same as my archived installer, returns the "too old" pop-up when I try to launch it.
'Tis a puzzlement!
Despite my penchant for archiving installers, and in view of the links included in the Apple doc and the fact that I can't conceive of a single use for them, I think I'm going to just trash the ones I've got other than Big Sur and Catalina (to which I'm thinking of reverting) and banish the mystery from my life.