You don't need an ECU to act as a rev limiter. Gasoline engines have had rev limiters in all different kinds and types for over 100 years. Some cut fuel, some cut spark, some cut both.
Even my push mower has a rev limiter, it just gets called a governor on that engine.
The majority of large American engines used the valves as the rev limiter. The valve springs were so weak that past a certain RPM the performance of the engine falls off so fast it isn't going to rev anymore, even in neutral. The valves never fully close, so performance is very poor. This is well below the mechanical limitations of the crankshaft, so nothing is really going to happen. Once interference engines showed up as common place in the market, it became mandatory to have a rev limiter. This happened as electronic ignition and fuel injection became more common, so it was logically incorporated into the ECU as time went on.
