https://arstechnica.com/information-technology/2019/03/boeing-sold-safety-feature-that-could-have-prevented-737-max-crashes-as-an-option/

The crashed Lion Air 737 MAX and the Ethiopian Airlines 737 MAX aircraft had more in common than aircraft design and the apparently malfunctioning flight system that led to their demises. Both of the planes lacked optional safety features that would have alerted the pilots to problems with their angle of attack (AOA) sensors—the input suspected of causing the Maneuvering Characteristics Augmentation System (MCAS) software to put both aircraft into a fatal dive.

The New York Times reports that both vehicles lacked an "AOA disagree" light—a warning light that indicates when the aircraft's two AOA sensors provide different readings—and an angle of attack indicator. Since the MCAS system relied only on one of the aircraft's AOA sensors, the disagree light and AOA indicator would have given the flight crew visible evidence of a sensor failure and prompted them to disable the MCAS. But both of these features were sold by Boeing as expensive add-ons. And many discount and smaller airlines declined to purchase them, as they were not required by regulators.