Not anymore.

See the Office related talks at CppCon 2015 about how to write portable code using what Microsoft calls the hamburger model, as part of their migration to a common codebase.

Basic a shim layer for low level OS APIs, the middle layer with the majority of code and another shim layer for UI APIs.

Which is exactly my point, the application architecture has to be done in proper modular layers to separate UI specific code that makes use of the UI elements that provide a good UX, from the common code.

Microsoft calls it hamburger design, and has held three talks at CppCon 2015 presenting how they unified their codebase across all platforms. They are available on YouTube.

Sure, but once upon a time developed from scratch, always supported PC, Mac and some UNIX variations, and the Web wasn't even a thing.