I can't imagine it would be very common. I'm not sure how that would work in most situations.

An ethical T would never have a session at a coffeeshop or something because that's a public space. The client might feel more inhibited. The client could be seen out in public with the T by anyone and that invades the Client's privacy. Even if the client didn't mind being 'seen' with a T, the risk might be letting the session be interrupted by something or someone outside of therapy.

A public space wouldn't foster the feeling of privacy and security that an office space does.

I also wonder if there would be liability issues involved.