green board has been used for many many years, yes, it is not as good by far as that new cementious board. I AM NOT SAYING AT ALL THAT GREEN BOARD IS OK TO USE, i am only saying it was used many many years before backer board was ever thought of and did its job.
I installed a new shower pan in my shower five years ago and I used green board and I have not had any problems.
I will not advise one way or another. I will catch enough flak on this answer anyway.