OneNote was a flagship WPF software as well. OneNote has announced it's all in on UWP moving forward.
WinUI included in the announcement above is the UWP XAML controls and their "XAML Islands" wrappers, so Microsoft is directly taking UWP open source contributions now, too.
So far as I'm aware, Microsoft's Ink controls have never had Win32 versions. For a few years they were System.Windows.Forms-only, for many more years they were WPF-only, and now the latest versions are UWP-only. That's a pretty core component part of OneNote. I don't think OneNote was WPF for all of its history, but there was certainly a time where a large percentage of it was WPF, from what I've heard. Which is also why I've heard OneNote led the vanguard to UWP and has been the Fluent Design 2.0 test bed for where Office's Fluent Design is moving, because it was the easiest to integrate UWP controls into the fastest.
My understanding is that most everything ink-related in OneNote is custom on top of the lowest-level APIs available. That tends to be Office's MO in general.
WinUI included in the announcement above is the UWP XAML controls and their "XAML Islands" wrappers, so Microsoft is directly taking UWP open source contributions now, too.