In the MCAS case the 'optional' component was an AoA display available to the pilots. Due to a misunderstanding about the design intent, planes without this option also did not display an AOA DISAGREE alert if the sensors disagreed. (I agree that such a simple situational awareness aid based on data the plane already has being a paid option is pretty absurd)

However all MAXes did/do have two AoA sensors, and prior to the fixes, regardless of the AoA display option, MCAS only considered one of them.