The term "apostate" really came into vogue in the org after the purge of 1980. Before that, you never heard very much about anyone leaving the organization voluntarily. It was more or less assumed that no one would ever leave the "truth" willingly, only that some people had to be df'ed because they didn't live up to the moral standards. The one exception was the "evil slave" class, and they had become rather antiquated too by the time I became a JW in 1969. However, the "evil slave" was pretty much restricted to the Bible Student groups that had separated from the WTS, and you hardly ever saw them anywhere. Once in a while they would show up and hand out tracts at a big convention, but that was about it.
By the late 1970's, though, a rather large contingent of "apostates" was developing. Many were non-anointed, thus could not be properly lumped with the "evil slave" class, and they were going off in a number of directions religiously, not just signing up with the Bible Students. In particular, a large number were embracing evangelical Christianity, thanks largely to the work of Bill and Joan Cetnar. So the Society couldn't just rumble about the "evil slave" anymore. That was a great term for Freddie Franz to use in his convoluted explanations of prophecy, but as I mentioned, you almost never actually saw an "evil slave". This new batch of ex-Witnesses was all over the place, though - virtually every congregation had a few, after the failure of the 1975 predictions. So they started tossing around the term "apostates," and it became the new buzzword that we all know and love.