"I've no idea how anyone could state...." The clue was several times a day, receiving ATC messages along the lines " ..high speed traffic two o'clock closing rapidly, no height information."
You need to think more about how limited your clue was. Indeed, your question could have been phrased
'in what circumstances do secondary returns omit Mode C height information?' or
'how many primary-only radars provided ATC services in the 80s and early 90s?' rather than your presumption that we deliberately or accidentally omit to select it.
I'm sure an air trafficker of the period will be able to help.