"We will typically not accept contributions that implement support for an OS-specific technolology on another operating system. . . We also do not intend to accept contributions that provide cross-platform implementations for Windows Forms or WPF."
Microsoft seems to be actually refusing to accept contributions that could make WPF cross-platform - it's a bit beyond "no official plans for cross platform".
"We will typically not accept contributions that implement support for an OS-specific technolology on another operating system. . . We also do not intend to accept contributions that provide cross-platform implementations for Windows Forms or WPF."
https://github.com/dotnet/wpf/blob/master/Documentation/cont...
Microsoft seems to be actually refusing to accept contributions that could make WPF cross-platform - it's a bit beyond "no official plans for cross platform".