Because, AFAICT, its sole purpose is for integrating complex numbers into std.format, but it is no longer being recognized by the current version of std.format, so it's basically useless. With the new overloads of toString, we fully support std.format.format(spec, c) for any complex number c and can handle any valid format spec, e.g., format("%5.2f", complex(1,2)), and so on. So there is no need for another function to format complex numbers using a format spec.