Also, if you didn't see it:Thanks, Ric. I should add that Silent Knight is an immensely useful bit of software; everyone should have this.
The unusual "software update required" dialogs are the same type as those discussed last year in the iOS 12 thread and described in a rather superficial Apple support article.
The update installs the "MobileDeviceOnDemand" package, which contains updated kernel extensions and frameworks to enable older systems to work with the latest versions of iOS. It also install a "CoreTypes" package containing updated icons for iOS devices.
If anyone wants a detailed listing of the latest package contents, drop me a note at josehill at gmx.com.