[hotpotamus]

Interestingly, Boeing had an indicator for an angle of attack sensor mismatch on the 737 Max, but it was an optional accessory.

[josephcsible]

At least that was an actual optional part, and not softlocking functionality the plane already had behind a paywall.

[derrikcurran]

Why is whether or not they pre-install the optional part an important distinction?

[josephcsible]

Because if they pre-installed it, then it was included with the purchase. Once you sell something to someone, you shouldn't get to charge extra money for using it.