Merge all implementations or have all of their programs use the same implementation? I find the former a little weird since there is, e.g., an MFC implementation and then a Win32 implementation (available as a COM object) and AFAIK those two are completely separate (i think the MFC predates the Win32 one and was made by some external company) and "modern apps" use XAML which is completely different from the MFC/Win32 stuff.
Bit of both, I believe. On the one hand, Office as the first Ribbon adopter has to maintain a lot of backwards compatibility (especially for add-ins), and so likely can't move all of its ribbons to a single implementation (though OneNote supposedly is using the WinRT/XAML control). Plus the React Native for Web control (Fabric UI) will always have to be a different from platform-specific implementations.
But the impression I have is that "soon" the MFC/Win32 one and the WPF/XAML ribbon implementations will both be marked deprecated and developers encouraged to use the WinRT/UWP/XAML one (which OneNote supposedly will continue to share moving forward), with Win32 (and WPF) developers expected to use XAML Islands.
(Where "soon" is some release milestone I'm fully not sure about. WinUI 3.0 General Availability, presumably? I think that's still scheduled for "second half 2020".)
