React Native already supports iOS, Android, and UWP. To add browser support all you need is something like React Native for Web. I made a small presentation a few months ago that shows this. Here's the source code: [2]. Take a look at the web folder.
Libraries like React Navigation [3] have also been built to support any platform that runs React code. It looks like Microsoft built yet another navigation library [4].
Also, check out React Primitives [5]. It aims to define a set of primitives that work on any platform that can be used to build more complex components. This is highly experimental, but I'm liking the direction where it's going, a unified React interface for any platform.
In addition, ReactVR is a great example of how React Native primitives can be extended to new emerging platforms [6].
Finally, React Native for macOS [7] answers the question that many have here about building native apps for macOS without relying on Electron.
[1] https://github.com/necolas/react-native-web
[2] https://github.com/migueloller/HelloWorldApp
[3] https://github.com/react-community/react-navigation
[4] https://microsoft.github.io/reactxp/docs/components/navigato...
[5] https://github.com/lelandrichardson/react-primitives