Along with a change to the new San Francisco font, iOS 9 now displays lowercase letters on the virtual keyboard by default when typing non-capitalized letters. This is a change that has been met with some criticism by users, but thankfully Apple has included a way to switch back to the all-caps keyboard.
Users can access the keyboard option by opening the iOS Settings app, choosing General, and then selecting Accessibility. Then scroll down to Keyboard, right underneath the new 3D Touch sensitivity options.
Once you've selected Keyboard, you'll see the first option underneath "Software Keyboards" is a switch for Show Lowercase Keys. While this is turned on by default in iOS 9, it's just that easy to turn it off.
In addition to the aforementioned 3D Touch options for iPhone 6s, a number of other settings that may be of interest to general users are also found within the Accessibility menu. They include the ability to turn off Reachability for iPhone models sized 4.7 and 5.5 inches, and the option to disable the "Shake to Undo" shortcut.
Disabling lowercase letters on the iOS 9 keyboard, of course, will not change the fact that the new system-wide font is Apple's custom San Francisco, making its way from the new Apple Watch. San Francisco also debuted this week on Apple's OS X 10.11 El Capitan update for Macs.