Both of my husbands and my father were Overseers much of my life. The three of them combined never received a dissociation letter. Writing such a letter may be cathartic to the author but probably won't have any positive effect on the elders who read it.
I just asked hubby whose job was the secretary, how he would have reacted to a DA letter. He said it just would have caused a lot of problems for the person who wrote it. He said what REALLY gets to them is those who successfully fade and are obviously happier than when they were active.
When we run into Jws we know we are always asked in that condescending tone, "How ARE you??" With the toothiest smile, we answer, "We have NEVER been so happy. We really are blessed! Blah blah blah. Like a giant weight has been lifted off our shoulders. Hahaha." Then we ask them about their family member who is the biggest screw-up. (there is always at least one) They absolutely can't handle it when you THRIVE after leaving.