Xamarin Forms already supports those platforms and you can use them today. The main stumbling block is that the tooling doesn't have project templates for those

Xamarin Forms already supports those platforms and you can use them today. The main stumbling block is that the tooling doesn't have project templates for those platforms yet. So you need to manually create the MacOS or WPF projects. Also, this new System.Maui looks like it will make everything in one unified project with platform specific code conditionally compiled in - at the moment Xamarin Forms is a bit more messy than that, requiring a project for each platform that share a common library to make the UI work on all platforms. The new stuff looks nice, so hopefully it'll be here on the schedule they mentioned (circa 2021)