This is interesting, I checked all the apps (that are running/enabled) on the two android devices in my household. Mine is set up with the traditional android UI triangle, circle, square, visible whereas my partner's has the "slide up" feature enabled where the back, home, and "background" buttons need you to slide your finger up or they aren't visible. Both seem to remain present and usable in apps, although the back button does cause "too much of a back avtion" in the third party camera app she uses, taking you home vs. back a step. These devices are both unlocked (but not rooted currently) devices activated on at&t firstnet. My coworker has a Verizon issued (came from the carrier) Samsung Galaxy and the triangle, circle, and square buttons are missing in a few apps, and totally inaccessible. I am not a fan of how much bloat cell carriers add to their android devices, so much so that I would say the coworker's phone is running a "Verizon Samsung version" of Android. I did a quick search and didn't find a full list anywhere of variations of Android by carrier or model phone, but I did find a number of users with similar observations about the extreme variation.