When I were lad (etc)........
Most pilots among rotorheads will remember R22s without governors.
We were lucky... corellators weren't available on early helicopters too, so it was another dimension that you had to keep an eye (or more importantly, a subconscious ear) on. I found that flyng a Bell47 concentrated the mind on rpm control.
Governors can hide carb icing, as they can open the throttle to keep rpm up without the pilot noticing, and the rpm only drops once max throttle is reached, at which point there is a real problem.