If contact is strictly letters or emails after a set period of time has passed, I think it would be harmless most likely, but it would depend on the individual client and therapist.

I know I'd like that with my current therapist and psychiatrist. Letters mostly. Mainly because I love fountain pens and stationary. But also because it comes across as more meaningful and you can tell more about a person by how they write.

Anyway, I've also thought about trying to track down my first therapist from college, just to send her a letter, especially around the holidays. I don't know if it'd be weird, but I also know I don't expect a reply and I'm mostly doing it for myself, to express gratitude and share with her how much she helped me and how much she meant.

Then, on the other side of the coin, because I work at a psych hospital on an acute, locked inpatient unit (direct care, not RN, MSW, PhD, MD, etc.) I can see a little bit of the other side. We actually have a rule too. I'd have to try to look it up to know specifically, but I believe it's a lifetime sort of thing, unless we grew up with them or were friends with them before they came in for treatment. Which kinda makes sense because we're at a hospital and not an outpatient office, there's a greater likelihood that people will want or need to come back at some point down the road.

In any case, I always encourage patients when they're discharging to write letters or call to let us know how they're doing. And I mean it genuinely. Stuff like that is always nice. I want to know that people I've worked so closely with are doing well. I just don't give out any personal contact info. If I give out anything, it's my work email so it's more professional and can act as documentation in case anything were to happen.