Carmakers started phasing them out by the late 60’s. In fact the only Camaros and Firebirds to have side window vents were the 67’s, first year of production.
I think designers thought the car looked better without them and anti-smokers knew that people flicked ashes out their exhaust side.
Right, the vent windows are the easiest way to tell a 67 from a 68 Camaro.