It wasn't in SR4a, which also did not forbid installing one when the other came as existing modification that thus did not take up the barrel mount. The restriction was in Arsenal, noting that weapons can be equipped with both a suppressor and a gas-vent, but if the gas-vent was on the suppressor did not give its benefits. Which is why it's possible the rule will get reintroduced with Run&Gun. However, given accumulating recoil, I'm not sure if they will.