The title does clearly talk about Max 9, so the Max 8 and 7 are irrelevant (the max 9 is longer, hence the possibility of needing an actual door for high densit

The title does clearly talk about Max 9, so the Max 8 and 7 are irrelevant (the max 9 is longer, hence the possibility of needing an actual door for high density setups). The title should still clarify that it's only max 9s with a plug door but as far as I understand that should be pretty much all 9s in the US: the additional exit is only needed in the densest configurations which are only used by ULCC, 85% of the global MAX9 fleet has a plug.