I've never worked with anyone long enough to have them retire and trigger the sending of this letter. I didn't even know there was such a formality and I work in the field.

Unless there are legal reasons for crafting such a formulaic letter, I'd be hurt receiving the equivalent of a mad libs goodbye.

I'd just assume share any thoughts or feelings at the last session and leave it at that. A letter, if genuine and personally written, would be much nicer (at least, imho) because then I could save it with my other keepsakes from both my own personal mental health journey and things I've received from patients at my place of work.