NT 3.1 (that is, the _first_ release of Windows NT in 1993) supported Unicode, at least as far as the UI elements were concerned (that it was UTF-16, well, ...)
Nope, UCS-2. Which is UTF-16, but just the BMP, so only the first 65k codepoints and all characters are fixed-length 16bits. Only later windows versions at some point learned UTF-16.