The newer documentation about creating a Settings.bundle on Mac Catalyst simply links out to the Documentation Archive:
The SupportedUserInterfaceIdioms key is documented:
Indicates that the element is displayed only on specific types of devices. The value of this key is an array of strings with the supported idioms. Include the string “Phone” to display the element on iPhone and iPod touch. Include the string to “Pad” to display it on iPad. This key is available in iOS 4.2 and later.
Is "Mac" a supported entry to specify a preference for the Mac idiom? It seems to work but I don't see any documentation for this.