Microsoft decided to mark the name without underscore deprecated, because those names are reserved for the programmer to choose. Implementation specific extensions should use names starting with an underscore in the global namespace if they want to adhere to the C or C++ Standard - or they should mark themselves as a combined Standard compliant environment, such as POSIX/ANSI/ISO C, where such a function then corresponds to one of those Standards.
Read this answer about getcwd() too, for an explanation by P. J. Plauger, who knows stuff very well, of course.