You can use a Unity Store asset called "Fullscreen Editor".
No idea why Unity hasn't got Fullscreen yet but only "Maximize On Play". It's very helpful if you're working on UI stuff cause you can test the game in Fullscreen without building the game, at least that's what i'm doing. And you can maximize any Unity Editor window with it's shortcuts, and supports multiple screens.
Also i noticed some weird "scaling" isues? when using "Maximize On Play" when working on the UI, because in 1920x1080 (FullHD) "Maximize On Play" scales the game to x0.84 due to not being totaly Fullscreen.