If you're referring to the AOA indication — they didn't though. Boeing sold the MAX to the FAA and the airlines as being equipped with an "AoA disagree" annunciator on every single one. It turns out that the annunciator didn't work unless you also purchased the AoA gauges (it's unclear to me whether or not there are gauges on the primary flight display or if they're just part of the heads-up display). That little glitch was considered too minor by Boeing to notify the FAA or the airlines.