This is the actual wording of the British Consulate reply on UK in Turkey facebook page as a response to a question posed by a Fethiye resident:
UKinTurkey
We are aware that several British nationals have received such letters from the Fethiye SGK office. We are under the impression that these are the original letters drafted in September which are now reaching home addresses.
We sent an official request for clarification to the Ministry of Foreign Affairs and SGK Head Office on 18th September and we are awaiting the reply.
We will, of course, let you know what response we get.
Until we have information to the contrary, we understand that the ruling applied at the end of May 2013 still remains-that is, the only circumstances in which a foreign national will be able to cease or withdraw their application from SGK is when they leave Turkey; join an official health scheme (like NHS); gain employment with a Turkish company that deducts SGK from their salary; or on death