Hi
Picking up this thread as seems to be the most relevant.
Firstly, the 'Enable App.OnStart' property no longer exists - the closest to this is 'Enable Navigation function in App.OnStart' which is being retired as we are now being pushed to use App.StartScreen.
The option to Run OnStart has disappeared in the last 2 weeks. I first noticed that it had gone when working within PowerApps 4 Teams yesterday.
The issue also applies to new apps created within normal Teams environments. If they are older than 2 weeks the option is there.

Newly created apps are missing the option.

We use OnStart a lot within our apps both normal and native Teams versions to set global variables etc.
Is this a temporary removal (bug) or something permanent?
If permanent what's the fix... having to save, close and reopen apps to push the changes through isn't a workable long-term option.
Cheers