I recall reading somewhere years ago now, that the rear ride height of the Z/28 model put the rear bumper height about 1" lower than on other Camaro models, thus requiring the large rear bumper guards.
However, the chart in 'Trim Heights' in the 1969 AIM (UPC0/A10) shows that other models (notably those with E78 x 14 and F70 x 14 tires) shared almost identical 'shipped' and 'curb' ride heights at the rockers (locations J and K) as the Z/28. Those models did not come with the V32 bumper guards, yet would have had comparable or even slightly lower rear bumper height to a Z/28.