Fantastic speculation from Allen Pike.
The nav bar is the strip at the top of the display that allows you to navigate between views. It features buttons like “+”, “Edit”, “Done”, “< Back”, etc.
iOS 11 has changed the design of the nav bar, moving the title from the center of the nav bar to its own line, left justified, and to a much larger, bolded font size.
Allen posits that this change was made to accommodate the disappearance of a physical home button and the corresponding shrinking of the bezel, the growth of the screen to just about the bottom of the phone.
In Allen’s view, the nav bar will move to the bottom of the screen, on either side of the virtual home button.
I think he’s on to something. Read the post, look at the pictures to get your own sense of this.