The answer is Easy Mode, accessible through Settings -> Display. I had assumed that Easy Mode meant "hide advanced settings" but it actually means "Make big icons and use a default home screen layout".
Nova Launcher has dozens of settings for its Home Screen layout and personalization. Included is a Notification bar setting, which allows the device owner to hide it on the Home Screen.
It can be found under the Look & feel settings.
Change the state of Show notification bar to experiment to taste.