The improvements were already mainstream before Chernobyl was built. It's all about making reactors tolerant to operator error, and if something does go wrong, make sure it goes wrong slowly, so operators have lots of time to react.

The RBMK reactor is quite an elegant design. Simple plug-and-play architecture for adding and removing fuel and other assemblies while the reactor was running, perfect for things like doping silicon for semiconductors and producing plutonium for weapons. But to get this capability you have to either give up affordability or safety, and they chose to give up safety.

The most obvious is that rods naturally fall into water now, which dampens the neutron exchange to a stop, which works automatically in case of power cut (The rest of improvements are about preventing water from overheating and forming H2, and preventing H2 from overpressuring the chambers or exploding, which happened in both Chernobyl and Fukushima).

Control rods aren't tipped with iron. Iron has a positive coefficient of reactivity.