I understand ,as Ians said, that if you were a major or above you could continue to use it ( my wife worked for an institution whose admin head was a retired major. He didn't use it, but because of his military attitude was known as "Major" behind his back!). I agree it has gone out of usage. I don't remember any of my father's WW2 vets generation referring themselves as Corporal or Aircraftman or Able Seaman! The deputy head of my 1950s school was proudly Lt Col although only a Territorial.
RtS