The previous 737-900NG-ER series also had this option for an extra aft emergency exit door behind the wing to enable high-density seating.
It is more of a hatch, that drops down, has a window, and is fitted with a slide.
This exit can be used when Ditching, unlike the 2 rear main doors that must not be used when landing on water.
It's not like the larger 757 Type 1 door (doors 3L/R) with a slide.
UK's Excel Airways were the first airline to order two of the -900ER type in 2006 for delivery for summer 2008, but the airline went under soon after.
Some airlines choose not to use this extra exit option on the 737-900ER, and on the 737 MAX-9 and will have it deactivated as Alaska have done here.
Although the actual door/hatch is still fitted, but when seen on the inside (in the cabin) it is blanked off with a wall panel with a window, and Pax would not know there was a door/exit there.
It can be reactivated if the airlines want it.
The A321N cabin-flex also has this option to use the extra aft exit for high density seating, along with a choice of overwing exit hatches too, which can also be blanked off if not required.