“oppostate”: “This will prevent their announcing anything about your not being a JW anymore, all you've done is leave that congregation and that's not a DF'ing offense.”
If that is all a person is seeking to accomplish – simply leaving or withdrawing from a particular congregation – then there is nothing else he or she needs to do but simply leave that congregation. There would be no necessity for any type of legal letter if that’s all that’s involved – leaving a particular congregation.
The only situation requiring a legal letter would be if someone wanted to formally leave the Jehovah’s Witness religion itself and no longer wished to be identified as a Jehovah’s Witness. But of course the tricky part is submitting a written disassociation while still expecting to get out of actually being disfellowshipped and having the standard “so-and-so is no longer one of Jehovah’s Witnesses” announcement made at the next mid-week meeting. Unfortunately, I think it looks like that would be a rather impossible outcome to avoid.
The only way someone can have their cake and eat it too by effectively leaving the Jehovah’s Witness religion and not being formally disfellowshipped is to just do the fade thing while simply telling anyone who tries to interrogate them that they don’t wish to discuss anything whatsoever. And if someone were to do just that – fade away while refusing to discuss anything – and they still got disfellowshipped in absentia, then they would probably have somewhat of a fairly good case for a lawsuit for basic breach of contract under “natural law.”