As I have posted on Twitter, App.OnStart property will be retired, and "alternatives" will also be provided as new App property.
Today, I discovered one of those alternatives is available in Preview environment - App.StartScreen property which work same as Navigate() in App.OnStart.
How it works
App.StartScreen property accepts Screen object - Screen1, Screen2 etc.
The screen you specify here will be displayed first when App launched.
Just as Navigate function in App.OnStart was, you can set Screen object using some PowerFx formula - like If/Switch statement depending on Param() value.
As far as I confirmed, StartScreen will not significantly improves performance when loading app, but it will improve readability of code in App object (fit and proper place).
I'm looking forward to more alternatives (especially,
Set()) will be available!