Windows Defender ATP Agent Onboarding State should include all supported ATP OS
In the Devices view, ATP onboarding state is only reported for Windows 10 devices. Other OS have a blank state.
Currently (Oct-Nov 2018) working on an ATP trial with a customer who has +90% Windows 7 workstations. Support for Windows versions previous to Windows 10 is currently a preview feature for ATP. Still, having the ATP onboarding state and related info for Windows pre-10, would provide for a much nicer overview. The customer doesn't want to hear to upgrade to Windows 10 all the time.
You got my votes. ConfigMgr should definitely reflect the state of ATP onboarding for all supported OSes.