"Support for Winforms 1.1 and 2.0 has been completed, and is now in a maintenance/bug fixing state."
"System.Windows.Forms implements its own driver interface to communicate with the host OS windowing system. Currently, we have drivers for X11, Win32, and macOS. These drivers translate the native window messages into WndProc compatible messages, to provide as much compatibility with native .Net as possible."
WPF is windows-only though.