First introduced in Android Marshmallow Google included a hidden menu with experimental options called System UI Tuner.

The option to launch System UI Tuner in Android 9 Pie has been removed by Google but the utility does still exist.

This application is simply a shortcut to launch the hidden System UI Tuner menu without the need to use ADB or install a custom launcher. This works the same as Google's own method of launching the UI Tuner in previous Android versions.

This will work on all stock AOSP and Pixel builds of Android 9. Third-party manufacturers may chose to disable this menu completely in their custom builds.

Note: Some features of System UI Tuner in Android 9 Pie are broken (such as hiding certain icons) there is nothing that I can do to correct this as it is part of the Android system. This app simply opens the stock built-in Android UI Tuner.