The problem appears to be that people have different opinions on what is an "essential" read-back and what is not. I believe that the ICAO lexicon does include the word "Readback" (after an instruction to an aircraft) where ATC require confirmation of a specific message received. One seldom hears that nowadays perhaps because read-backs of just about everything is the norm.
My long gone Uncle - a former Spitfire pilot - told me that on being attacked by the Hun his mates would sing out "Break right" in urgent tones. Now that is what I would term a most essential call. But he never said they had to read it back.....
Perhaps this shows that what is an "essential" call is in the eye of the beholder..