Use of undefined constant MODULE_PAYMENT_AUTHORIZENET_AIM_CURRENCY

[DrByte]

So settings are ultimately, intended, supposed to be in a perfect world, remembered when uninstall/install USPS, FEDEX, UPS, Auth.net AIM/SIM, Square, etc during an upgrade?

No, not "when uninstall/install", because "uninstall" removes the settings.

The REASON for the historical need to uninstall/reinstall was just to "add any new config settings for that module" if any were added, else the new features wouldn't work or maybe even errors would happen unexpectedly. That's it.
So the newer coding approach has been to try to ensure that if the module is missing a new config setting then just push it in quietly so there's no need to uninstall/reinstall. When we code it properly it works great. In this authnet case we missed one line of code to empower it. Argh.
But still, if you uninstall the module, you will indeed lose its settings and have to re-enter them.