App Store on High Sierra suggests Mojave-only updates
The current latest version of the three iWork apps (as mentioned in the question), are supported on macOS High Sierra 10.13 and iOS 11. You are right, that the apps as well as the OS will be well supported into the future.

1. Pages

 



2. Keynote

 



3. Numbers

 



However, as mentioned in the release notes for the apps, certain features require the app to be running on macOS Mojave and Siri shortcuts requires iOS 12 which is the first iOS version to support this feature.


Pages for macOS
What's new in Version 7.2



  Support for Continuity Camera allows you to take a photo or scan a document with your iPhone and have it automatically appear in your document on your Mac. Requires macOS Mojave and iOS 12.



Pages for iOS
What's new in Version 4.2


  Support for Siri Shortcuts. Requires iOS 12.





Keynote for macOS
What's new in Version 8.2



  Support for Continuity Camera allows you to take a photo or scan a document with your iPhone and have it automatically appear in your presentation on your Mac. Requires macOS Mojave and iOS 12.



Keynote for iOS
What's new in Version 4.2


  Support for Siri Shortcuts. Requires iOS 12.





Numbers for macOS
What's new in Version 5.2



  Support for Continuity Camera allows you to take a photo or scan a document with your iPhone and have it automatically appear in your spreadsheet on your Mac. Requires macOS Mojave and iOS 12.



Numbers for iOS
What's new in Version 4.2


  Support for Siri Shortcuts. Requires iOS 12.