What is the proper or closest TERM environment variable value that covers the Windows 10 command line/terminal window?
For the traditional Conhost console window (native Windows 10), there is no ncurses terminfo definition at all. As of current Windows 10 versions, it is somewhat xterm-compatible.
For the Windows Terminal app (available through Store or GitHub), latest ncurses versions have
ms-terminal, although the app is Xterm-compatible because pretty much everyone uses