I am pleased that I did not write one, though I went through a stage of wishing to so so much, so that I could live my life without looking over my shoulder.
The problem is that I know I would still have "looked over my shoulder" in that I would not want the Dubs saying stuff like "he only left so he could put up a Christmas tree" etc etc
As it is I am free to speak to any member of my family who is a Jw, and any of the old JW acquaintances.
I am often able to plant a seed , and I am in a position to be really supportive to any who wish to fade.
I do not recognize them as having the right to my resignation, I did not join the religion as it is today, I was baptised in 1962, it is nothing like it was then, nothing. (Well, except being wrong on everything, they were then as well)
If I were going to write one I would be tempted to do a Groucho Marx: "Dear Sirs, I wish to resign from your religion as I do not wish to be a member of a religion that would have someone like me ".
Groucho wrote that to a club, not a religion, the problem with even that bit of piss-taking is that the cult-think would kick in and they would take it as a confession of gross sin.
My advice is do not give them the satisfaction of closure, they do not deserve it, remain an inactive one, even if you are silent, by your non-support you are pointing the finger and saying there is something wrong with the religion, by following their rules and DA'ing you are admitting to them ,in their mind, that there is something wrong with you.