I do not see the depositing of a dated, signed, witnessed Letter as wrong, or even not necessary, you may need it, if you never do, that is good.
It is still not a fool proof way of leaving, with no further action by Elders guaranteed. They can still claim, years later, that you Disassociated yourself, quite accurately, and then you have the Announcement, and the effect upon you is the same as if you had been DF'd.
But, who knows ? it may give you a number of options, especially one that would work even without the Letter, which is to say that if any Announcement is made, or any rumours spread, you will Sue the Elders for Defamation of Character, the threat, even if you are in fact in no position to Sue, may be enough to get them to back off.
I know of one guy who did this, an Elder told me had done it, and said " So there was nothing we could do".