You made a comment asking about whether the letter is about/for him or you. I think the answer is it doesn't matter. If you do it to make you feel good, what's the problem with that? If it happens to make him feel good too, great!

You should do whatever feels right to you. I have written letters to a former T about 3 times per year since we terminated about 5 years ago. I have had a new T this whole time (I moved, that's why I had to stop seeing T1) but I still get some comfort from touching base with her. She seems to enjoy it too and almost always responds.

Best,
EJ