This mode declutters the interface by hiding all of Sizzy's UI elements such as the toolbar, bottom bar, and debugging panel. Unlike the modes discussed in View Modes, Zen Mode is not standalone—it's something that you use in combination with other modes such as Responsive Mode or Focus Mode.
This feature does not have a dedicated button in the UI. You can toggle it by pressing Z on your keyboard.
You can pick and choose which UI elements to hide in Zen Mode by going to Settings -> General -> Zen Mode.