If I set the debugging options on Android not to allow background processes and press the Home button, the Xamarin Forms app is killed and App.OnSleep is not called as expected and documented.
However, the Android activity OnPause is called as expected. (OnStop is not called as expected when the app is killed due to resource constraints.)
This is really unfortunate behavior, since the Xamarin Forms app lifecycle events cannot be used to reliably save the state of the app.