I find it weird that somehow 1600x900 isn't an option for windowed mode, considering that there are 16:9 modes in other dimensions and it's available on pretty much every game with adjustable windowed mode size.

I prefer to use 1600x900 because it fits the window nicely with the edges of the background visible so I can see other notifications, while still big enough to see the things in the game. Not having 1600x900 is quite inconvenient for me.