/etc/shadow was a thing on SCO Unixen as far back as 1991, to the best of my memory (and maybe sooner -- but not Xenix, which was System 7 based and discontinue

/etc/shadow was a thing on SCO Unixen as far back as 1991, to the best of my memory (and maybe sooner -- but not Xenix, which was System 7 based and discontinued after 1991).