How even does a "plug" door blow out like that? That's a seemingly very robust design where cabin pressure holds the door in place passively, in addition to the latches holding it in place.
A bit like when you buy a car model that doesn’t have all the options installed, there’s a space for the button or switch for the option in the dashboard or whatever but it’s filled in with a fixed bit of plastic - it saves them from having to produce multiple different versions of dashboard, or in this case plane fuselages.
I think it is indeed new for these Max planes and the airline purchased this one in this configuration that would not require it (fewer seats), and adding a functioning exit in (including life rafts and slides) would simply cost a lot more (and weigh more) than just blanking it out with a piece of metal. Not to mention extra maintenance and testing of said exit.
The airline may have the desire to buy a model with fewer seats because it’s cheaper and weighs less, but might also have a requirement for fewer cabin crew members too.