JEHOVAH'S WITNESSES - What are JWs taught happened in the "Great Apostasy of 1980"?

As has been said, there was nothing particularly noteworthy about that apostasy, other than a member of the Governing Body was involved. Apostasy rears its ugly head from time to time and it's rooted out, just as it was when the apostles ran the show (compare 3 John 9-10).