I've gone NC with my sister. It's been two years, and so far so good (or great!)
I obviously feel bad for my DM but I've told her it's not really her fault.
It might be the season, but would it make sense to write a final letter? Or simply there's no point?
I don't ever want to see/talk to her again, so from that POV I don't see why it would make any difference.