I know you can find it in View -> Fullscreen (or using the shortcut), but I miss the little fullscreen-toggle button that 2.8 has.
Is this really something you’re toggling so frequently that you need a dedicated button for it in the GUI?
Honestly, yes. But I can live with the menu.
+1
you can also use Alt+Return
I didn’t even knew fullscreen mode was still available 
I thought the button was present because the title bar function menus on OSX disappear when you go full screen. (there alt-enter will then be the only way to go back to windowed mode)