It still doesn't work on older Windows, which seems like a glaring omission to me. Windows 7 remains the most popular version of Windows; UWP runs on Windows 10+ only.
All of the official ports of React Native are targeted at mobile or touch-first platforms. (I consider the UWP and Ubuntu ports official in that they come from Microsoft and Canonical, respectively.) Only the unofficial Mac port can truly be said to target a desktop platform.
What underlying toolkit would a Windows desktop port of React Native use, anyway? WPF? Old Win32?
Microsoft has already answered this. Their React Native target for the windows desktop uses UWP.
Microsoft is pushing UWP as a capable desktop platform. AFAIK, they aren't ending support for their other platforms just yet. WPF is still being worked on last I heard. If you look at win10+, UWP is a perfectly valid choice for React Native. You could have React Native targets for WPF, win32, etc etc if you wanted.. I'm not sure what use cases could benefit from that though.
Keep in mind, if they're missing something in UWP that they need for React Native, they can add it to UWP.
