I have a vague recollection of my 20 floppy of Slackware already having /etc/shadow. That would have been fall of 92 or winter 93, based on where I was living at the time.
I think they may have been mounting parts of /etc as well. People get the idea that managing accounts for a cluster of boxes should be centralized. It's all fun and games until the network mount disappears.
Can't remember exact date, but might have been around time of SVR4 intro.
I know because I remember going "ugh", but without investigating the reason why it (shadow) was introduced :) - which was of course wrong on my part.