Baptism is not a legal status. It is no contract that is legally enforceable. Most people would call it a promise and promises alone are not contracts. Contracts are enforced by a court yet in North America and Western Europe no court will decide an internal religious dispute. My thinking of the reason for a letter is its pscychological impact. Just by giving it them, you are foolishly acknowledging their legitimacy. No one has ever been fined or imprisoned for walking away from JWs.
There is no legal basis that I can think of for the letter. All you did at baptism was make a promise, one that is rescindable at will.
The legal question is clear. What makes it difficult is the emotion toil and potential loss of famiy and friends. Perhaps we should call it a form of social contract. It is clear what the baptizee promises but what does the baptizer promise? I'm not aware of any negotiations. I view baptism as affirmation of a particular creed. What it means must vary with denomination.
No legal duty but how I remember the threat of loss of love.