Analysis of information sources in references of the Wikipedia article "Windows Forms" in English language version.
Windows Forms is continuing to be supported, but in maintenance mode. They will fix bugs as they are discovered, but new functionality is off the table
WPF is not intended to replace Windows Forms. [...] Windows Forms is still alive and well, and will continue to be enhanced and supported by Microsoft for years to come. WPF is simply another tool for Windows desktop application developers to use, when appropriate.
It is very unlikely that the implementation will ever implement everything needed for full compatibility with Windows.Forms. The reason is that Windows.Forms is not a complete toolkit, and to work around this problem some of the underlying Win32 foundation is exposed to the programmer in the form of exposing the Windows message handler
Support for Windows Forms 2.0 is complete. At this point, we are largely just fixing bugs and polishing our code.
Does Winforms run on OSX? Yes, as of Mono 1.9, Winforms has a native OSX driver that it uses by default
For tools that are mostly OpenGL/DirectX based, use Windows.Forms, keeping in mind that some bug fixing or work around on their part might be needed as our Windows.Forms is not actively developed.