On board the aircraft, they would have applied the changed product rule.
When they built the new plane, they had to take an inventory of everything new on board. When they added the MCAS, they had to establish the criticality of the system. This would have been classified as minor.
This means that, if the failures were major and the system was faulty, they would have slightly increased the pilots' workload.
In my opinion, that's where the mistake was made; it was misclassified. They took it for granted, like the trim system in general—