Old Zac had nearly twice the amount of damage post-6 compared to current Zac. So many people seem to forget that Zac's old ult would hit 3 times for the same amount of damage. Which was A LOT of his overall time.
His old Q also had a MUCH higher base damage, giving him more pressure during ganks (pressure he now gets with the CC on his Q), AND the slow lasted 4 times as long (2 seconds vs. 0.5 seconds now) AND it has significantly lower cooldown at low levels.
Don't pretend that Old Zac didnt pay for what current Zac now has. He may be overtuned, but saying to just play like before is ridiculous and not possible, because without the CC on his Q (and old damage from his ult) He is flat out worse in every aspect.