With Android 9 Pie, Google has included a new feature in Developer Options. And that is "Feature Flags". This is pretty similar to the flags that you'll find in Google Chrome. It will enable or disable some features within the operating system, however it is important to note that these are also experimental, and fairly limited right now. But if you are looking to test out some of these flags, and perhaps rollback some of the features that are new in Android 9 Pie, here's how you can do just that.
To get started, you are going to need to enable Developer Options. This is done, basically, the same way as in older versions of Android. You'll need to head to Settings, tap on System, then About Phone. Scroll to the bottom and find the build number. You'll need to tap the build number nine times. Then you'll have Developer Options enabled. Afterwards, hit the back button, then tap on "Advanced". You'll find the other system settings, as well as Developer Options there. You'll find Feature Flags under the "Debugging" heading there, which is about a third of the way down the list.
As of right now, there are only seven flags available to be turned off or on in Android Pie, which mostly include changing the layout of things in the settings. The first flag is settings_about_phone_v2. This flag is enabled by default. Disabling it will essentially just bring "About Phone" to the top of the list, and move "Backup" to the "Advanced" part of that screen. It doesn't really do a whole lot here, but it does work to move the About Phone settings to the top, making it easier to get to.
The next flag is settings_bluetooth_while_driving which is set to "false" by default. But turning this on will add a "Use Bluetooth when Driving" toggle in the Connection Preferences part of the settings. Even with the flag turned on, that toggle will still be turned off by default. But turning it on will automatically enable Bluetooth while driving. This is a good feature to have if you do use Bluetooth for music while you are driving.
The next few flags don't actually do anything yet. That might change in future versions of Pie or in Android Q next year. But settings_data_usage_v2, settings_audio_switcher, and settings_systemui_theme all do nothing as far it goes right now. With settings_zone_picker_v2, it is set to "true" by default and will show a more detailed view when you have "Automatic Time Zone" turned off in the Date & Time settings. Turning it off will essentially bring it back to what it looked like in Oreo. This is going to make it look cleaner and less cluttered.
Finally, the settings_battery_display_app_list now effectively does nothing as well. In earlier betas of Android P, this would bring back a list of apps and system resources using your battery. But in the stable version of Android 9 Pie, it now does nothing. It's unclear if this will be fixed in a future update to Pie, however. These Feature Flags in Android can really make a difference in the long run, but as far as things go right now, there's not many that actually work or do anything, and it's not even worth messing with just yet. But it wouldn't be surprising to see Google add more flags to this section in future versions of Android.