JOHN DOESN’T KNOW THIS IS GOODBYE FOREVER.
“How long?” he asks, and Sherlock says, “Six months.” “And after that?” “We’ll see.”
Sherlock leaves, leaving John with the impression that this is not necessarily the end. CERTAINLY not that Sherlock is going off to work on a mission that Mycroft has calculated will mean his death.
NOBODY FUCKING TOLD HIM. Not Sherlock or Mycroft, not Mary (who you can bet knows what’s up, because she probably still hears things here and there). They just left John to believe that of course Sherlock will make it back at some point.