Every Microsoft operating system going back to early versions of MS-DOS can use / as the separator.
This is not necessarily true of the "user space" of Microsoft systems. Paths are not only traversed by the "kernel", but also manipulated by applications like the Explorer shell, which may or may not like forward slashes.
It's partially the opposite. Some system calls only accept backlash, but user space programs and libraries may abstract this. E.g. LoadLibrary, which is similar to execve/dlopen.
Indeed. "Windows API" is very broad; it encompasses all sorts of user-space middleware, some of which could very plausibly munge paths in ignorance of forward slashes.
This is not necessarily true of the "user space" of Microsoft systems. Paths are not only traversed by the "kernel", but also manipulated by applications like the Explorer shell, which may or may not like forward slashes.