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. Any features that the UI Tuner lacks are not something that I have control over, please contact Google if you would like to make a feature request or suggestion.
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.