Thank you for your reply and I’m gad you found the article helpful :).
I believe the letter is for your closure and not for the other person. I also believe that if there’s something you need to say to someone, don’t leave anything unsaid.
Part of closure can be knowing you did all you can and said all you could say — then leaving it there. As long as you feel you won’t be anticipating a response from the other person, I think it’s ok.
I would really check yourself first and see why you want to give the person the letter. If it is because you know you will have peace when you have said everything you need to say then I think it would be alright to send it. If you are trying to make the person feel bad or hoping for a reply, I would encourage you to write it and leave it unsent.
Only you know your true motivation so just do some soul searching and discover your true reasoning for wanting to pass it on.
Hope that helps!
Love and blessings,