I tried to find some information for you on my 2001 watchtower cd rom but I can't seem to find when this arrangement came into place. I did find a reference in the Proclaimers book.
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jv chap. 8 p. 103 Declaring the Good News Without Letup (1942-1975) ***Jehovah’s Witnesses were not unaffected by the sexual revolution of the 1960’s. In fact, several thousand—a small percentage of their total number—had to be disfellowshipped each year, the majority for sexual immorality. With good reason, then, Jehovah’s people were given direct counsel at a series of district conventions held in 1964. Lyle Reusch, a traveling overseer originally from Saskatchewan, Canada, recalls the talk "Keeping the Organization of Public Servants Pure, Chaste." Said Reusch: "Frank, straight language on morals spelled things out in plain talk."
There are a lot of watchtower and awake articles referencing disfellowshipping. From what I have experienced and the 30 years I was in, I have not noticed much of a change in their view. But there are others here who might have a better idea. My father has told me that in the early 70's it wasn't a disfellowshipping offence to practice homosexuality and later that changed.
I haven't read it yet but the Crisis of Conscience by Ray Franz is supposed to have some info on when it came about and why.
Anyway, I am not much help but thought I would put in what I could find and remember.
Blondie will probably have lots of info for you.
