Supported, just but it's a second class citizen, for example, no tab completion of directories on the cmd prompt when using forward slash. Some commands will not except it;
e.g. type c:\home\foo.txt -> cats it out
type c:/home/foo.txt -> 'The syntax of this command is incorrect.'
whereas cd will except either forward or backslash
Now obviously any Windows port would have to call Win32 APIs instead of POSIX APIs (unless you're writing for a Cygwin-like layer).