Wasn't the second AOA sensor an optional upgrade? Seems it should be mandatory for a start. Edit: It had 2 AOA sensors but the indicators showing each reading

Wasn't the second AOA sensor an optional upgrade? Seems it should be mandatory for a start. Edit: It had 2 AOA sensors but the indicators showing each reading and the disagree light if they differed were optional (https://www.nytimes.com/2019/03/21/business/boeing-safety-features-charge.html https://www.nytimes.com/2019/03/21/business/boeing-safety-fe...)