But isn't winforms pretty much legacy? Microsoft has been encouraging the use of WPF.

Looks fine to me! https://github.com/dotnet/winforms/pulse

I don't know much about WPF, but it seems to be in an odd place with UWP. The fact there are no plans to port WPF MacOS/Linux, combined with the increasing emphasis on cross-platform... I'm curious where it goes from here.

Xamarin.Forms is not Win Forms AFAIK.

Legacy or not, Win forms would be better then nothing. For that to happen GDI+ need to be ported to core and for WPF to happen DirectX needs to be ported to core. Its all into UWP now.

So I guess it will not happen soon. MS should reboot IMO.

GDI+ is available on .NET Core as Mono.Drawing, I think. Windows Forms is tied to a lot of Win32, though, so GDI+ alone isn't really enough.

I mean that the Mono project has already ported WinForms to macOS and linux.

That is not the same, there are other cross-platform libraries. Give it OTB.