You want Avalonia for that: https://github.com/AvaloniaUI/Avalonia

I have really high hopes for Avalonia but don't know anyone who's used it for anything other than kicking the tires. I'd love to hear of any experiences.

And I really hope MS is neglecting WPF because some key insiders are looking to hand over the desktop story to some combination of Avalonia and Blazor.

I am using it for not trivial vector drawing app [1], there is even IDE [2] written using Avalonia. Generally its really great way to write GUI applications (currently for Desktop). [1] https://github.com/wieslawsoltes/Core2D [2] https://github.com/VitalElement/AvalonStudio

They have a directive to move it to .net core - assume I assume this will make it cross-platform?

The repository for WPF for .NET Core is linked right here at the top. You can explore it yourself. It only supports Windows (there's a lot of hardcoded Win32 stuff in it). Microsoft has said they won't spend resources to support cross-platform efforts on it. They also likely wouldn't stop community forks from trying (just don't expect MSFT employees to participate during day job time), but again you can explore the repository here and get a sense of how much work it would take (a lot).

.NET Core has always had platform-specific libraries that only work on a single platform or work somewhat differently on different platforms. The UI libraries (WPF and WinForms) are just the biggest examples now of platform-specific libraries on .NET Core.

That is like saying C++ is cross platform so MFC is cross platform as well.

WPF relies on Windows specific functionality.

MFC used to be cross platform (Windows / Mac OS System 7) during the early days.

Nope, you can't build WPF apps on Mac using Visual Studio for Mac.

Actually C/C++ is cross platform that means if windows is written in C/C++ Windows is cross platform.