December last year when Havoc launched, the only (main) modifers were Cranial, Pus, Moebian, and Blight. Last March it got updated to be the current rotation we have now, alongside the balance and flow changes; slightly easier shooters, tons of elites, monstrous cooked into it, and twin spawns.
They said this about updates to Havoc:
So ideally we’ll get new modifiers and general changes to Havoc whenever their next big update is.