Debbie,
I have had close friends ghost me, stopped returning my texts, etc. Everything is temporary. The Buddhist Four Nobel Truths talk about clinging causes suffering.
I get angry and I mourn for the loss of my friends. This is life. As Byron Katie said, If you argue with reality, you lose.
It cannot hurt sending a letter of closure. I would caution you that you may get anger or blame or at least being defensive from them.
I am of mixed feelings on “closure.” What if you did send those letters? What if they never responded? What if they did respond but with hostility? Would any of that make you feel any better?
Mark