Game Streaming native app for Xbox One?

The only platform you can stream directly from the XBox for is Mixer.

This is incorrect. You can stream to Twitch from the Xbox using the native Xbox One app.

That’s not possible except for Cloud Streaming. Steam Deck runs Linux, and the Xbox app doesn’t support PC streaming.

You’ll have to install Windows to play Windows Store games on Steam deck natively or stream using a third party solution from a Windows PC

Not natively, but if you use the Xbox App to stream the game from your console, you will be able to use an Xbox controller connected to the PC to play Red Dead Redemption 2. This will still be using the console, but you can stream it to the PC.

Details on how to stream to Windows 10 via the Xbox App can be found here, if you don't already know.

Although Twitch tell me they cannot support this issue and MS tell me it's a Twitch issue: I don't think Twitch values Xbox native-Twitch app streamers. They don't bring any where near the volume of traffic to Twitch that others do. Anybody who streams professionally i.e. generates a revenue for Twitch and themselves tends to use dedicated capture hardware. The bad news is, this isn't a priority for either company.

Has anybody tested native-app streaming with one of the other providers?

Xbox doesn't have any VR headset that works native on games, I think you can use the PC and the streaming to PC but this isn't native.