Rumors about this have circulated for a while, but there is new confirmation from Southwest Airlines and the Wall Street Journal that not only was a key safety feature disabled on the 737 Max aircraft, but Boeing didn't make it clear to airlines that it was disabled.
According to Southwest, Boeing informed them of the non-operational status of the "angle-of-attack disagree" warning light, which would otherwise alert pilots to a malfunctioning of the plane's angle of attack sensors, after the crash of a Lion Air flight in Indonesia. More critically, says Southwest, the plane's manual did not mention that the warning light had been disabled.
The manual documentation presented by Boeing at Southwest’s MAX entry into service indicated the AOA Disagree Light functioned on the aircraft, similar to the Lights on our NG series. After the Lion Air event, Boeing notified us that the AOA Disagree Lights were inoperable without the optional AOA Indicators on the MAX aircraft.
The "optional AOA Indicators" are a feature Boeing asked customers to pay for as an add-on package. It's not clear how pilots would know which aircraft had the feature installed, and which didn't. In response to two fatal crashes and subsequent removal of the 737 Max from service, Boeing is now making the feature "standard" and are retrofitting each plane at no cost. It is thought that a malfunctioning angle of attack indicator, coupled with anti-stall software that automatically noses the plane down with no pilot input, was the cause of both crashes.
The Wall Street Journal confirms that not just Southwest, but Federal Aviation Administration safety inspectors and supervisors were "unaware" of the deactivation of the safety feature. As Boeing faces federal investigations and furious shareholders, the company's decision to upsell what now appears to have been a vital safety feature—without adequate warning to pilots on planes with nonfunctional systems—is going to be deeply probed.