I never received similar, maybe because I never ended with my therapists sharply. I emailed back and forth with both for a while after stopping going to sessions and the communication just gradually died down.

To me, these sound like some pretty conventional personalized business letters that many different kinds of providers would send to their long-term clients. I do myself sometimes., of course those sound different as my business is not therapy and is not focused on personal stuff much, but I do add individualized elements. I think it is just common courtesy and a way to keep the line open to potential future work by making it so appreciative/positive.