I don't think it is acceptable, especially if, after announcing it, the client expresses objections. I think I personally would not have cared much about it in therapy and I terminated my own therapies that way (telling the Ts just via email), but I certainly would not like if a medical doctor, lawyer, work colleague did this unexpectedly, in the middle of some ongoing, unfinished work. I would also never just quit a job from one week to the next, without letting the others involved know a few months in advance, without making sure things are organized properly and former clients/collaborators can also access me later in case they have questions. I had some colleagues quite unexpectedly quitting and disappearing though and it can be very messy and stressful even in work that does not involve much personal emotional material. I think it is unethical and irresponsible in every long-term collaboration, and therapy is often more sensitive than any other professional connection.