The rationale talks about easy portability to Windows, but I'm skeptical - the underlying API and filesystem is different enough that this is likely to cause problems. '\' vs '/' to start with.
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
I write a lot of single source Windows/OSX/Linux utilities and the nice thing about Windows is that back and forward slashes are interchangeable so you can just write for POSIX.
Most languages (including Rust), already support path libraries that abstract this away from you. As long as you use these libraries properly, you should never have these issues.