As far as anyone could tell, MCAS was not encountered before in the US.

Yes, there are many reported incidents from US pilots of stability trim incidents in the FAA's database which, in hindsight, were due to MCAS. The pilots couldn't identify them as such at the time because, of course, they didn't know MCAS existed. But they were able to figure out the corrective action in time to maintain safe flight. (The reports also criticize the flight manual for being inadequate.)

https://www.google.com/search?client=ubuntu&channel=fs&q=us+...

Also all of the US-based MAXs had the optional AoA Sensor Disagree indicator, which would instantly show the source of the issue. The international pilots were in the dark because Boeing turned a critical safety feature into a revenue stream.

AoA Disagree was supposed to be turned on on all 737 MAXs, but it depended on AoA indication, which was an option. This was a mistake Boeing admitted to and has since corrected.

Options are not all tied to fees, the process of ordering an airplane is quite complicated because of the immense amount of customization each airline does before they even pay for the aircraft. Its unclear how much this option cost, if it cost anything.

The reason why AoA indication is an option is that it requires more advanced training for the pilots, some airlines provide this training and some do not. The airlines are the ultimate decider on how much training pilots will get, Boeing and the FAA can only set minimums.

Do you have any official sources to support anything you're saying? It seems unbelievable to me that Boeing would introduce MCAS, a complete flight characteristics "augmentation" system and pass it off as a standard 737 without any special training, but would only offer AoA indication as an option due to training.

Most of those reports involved the autopilot or were while flaps were extended, which disables MCAS. The ones that did not were complaining about not being informed about MCAS and not about it actuating. There were even incidents reported as being MCAS that did not involve stabilizer trim or a 737 MAX.

Reports like the one where the plane started descending when autopilot was engaged are still relevant because they indicate issues with the angle of attack sensor, which is also what MCAS relies on.

AoA sensors are notoriously unreliable and Boeing should not have depended on just one.

Yes, definitely.