Any implementation of this would have to be an optional interface setting, with the default remaining at the top. For as many people who really like the idea of having the app bar at the bottom of the screen, there are twice as many who really don't want it there.
For those who might not know, it is very complicated to build an interface as tricky as Privacy Browser's that has the option of having the app bar in either location. For example, the scrolling of the app bar is designed based on the webview and the app bar scrolling in the same direction. Scrolls are passed first to the app bar and then on to the WebView when the app bar is done with them. Moving the app bar to the bottom complicates the system because the app bar would need to scroll in the opposite direction as the WebView. It isn't really obvious this should even work from a GUI perspective. Perhaps it would should simply appear and disappear if it is at the bottom. However, coding that is complex, especially trying to avoid these types of security scenarios: https://jameshfisher.com/2019/04/27/the-inception-bar-a-new-phishing-method/