Shunning is probably as old as disfellowshipping in the WT story, but the rules were definitely softened for a while by the August 1, 1974 Watchtower.
The reason behind shunning
When someone was announced as disfellowshipped or reinstated, it would get so quiet in the meeting. What it did was make me realize (then) that the R&F had so much power that they could really put us in line spiritually. It led me to believe that Jehovah was working through them whenever they exalted their power.
Of course, looking back, there was NO reason for this other than the cult brainwashing tactic to believe whatever they say.
I heard of an experience where the person being disfellowshipped was brought on stage and then their crimes against the witchtower were announced, this was years ago.Totaly crushed that person into submission I'll bet.
Kind of like the Salem witch trials.