The forward slash (/) as a directory separator has always been acceptable to NT. One source says that it's been acceptable to DOS since DOS 2.0, although I don't have a lot of backing on that.

The only place that / as a directory separator doesn't work is when interpreted by cmd, as it's then ambiguous with DOS-style switches. And even then, modern cmd tries to understand / properly when possible. Most of the time it works as long as it is not the first character of a path (which you would not frequently see in Windows anyway), as there's then no way to tell it apart from a switch with a long name.

The canonical representation of paths in Windows uses a backslash, but 95% of the time the slashes are interchangeable.

IIRC (it's been a while), the native NT API does _not_ accept backslashes; win32 converts forward slashes to backslashes when passing along filenames to NT. The distinction doesn't matter very much, though, since very few people use the NT native API.

> One source says that it's been acceptable to DOS since DOS 2.0

I can confirm that from personal memory.

At the time, the biggest vendor of UNIX-like systems was Microsoft (Xenix), so compatibility was a benefit. They soon sold that product off, though.