SystemUI Tuner is a secret menu that was first introduced in Android Marshmallow (6.0) but in Android Pie (9.0) the option to launch it has been removed. Pixel Tuner is simply a shortcut to launch System UI Tuner's secret menu without the need to use Android Debug Bridge (ADB) or install a custom launcher.

Features (may vary depending on the phone used)
• Ability to control status bar icons by showing or hiding them (controllable icons are those of rotation, headset, work profile, screen cast, hotspot, bluetooth, camera access, do not disturb, volume, wi-fi, ethernet, mobile data, airplane mode and alarm)
• Ability to show battery percentage always or only while charging (especially useful if the option is not present in your phone's settings)
• Ability to hide the clock or add seconds to it
• Ability to show low-priority notification icons (by default, notifications you’ve marked as low-priority don’t appear at the top-left of your screen)
• Ability to activate do not disturb mode by setting the volume to zero and holding the volume down
• Ability to activate ambient display to see basic information even when you’re not using the device

Important notice
You can uninstall this app once you make any changes, you won't lose them. However, to restore the initial situation you will need to install this app again to be able to open the SystemUI Tuner's secret menu.

Why is a feature missing?
Features that are missing from the SystemUI Tuner are not something I have control over, they are the ones that your phone manufacturer has chosen to implement. Also, some SystemUI Tuner features are broken (like hiding certain icons), there's nothing I can do to fix it, as it's part of the Android system.

Compatibility
The Pixel Tuner will work on all stock AOSP and Pixel builds of Android 6+ and may work on most phones out there, however third-party manufacturers may choose to disable this secret menu completely in their custom builds. I have no control over this and cannot add the secret menu to your system, only your phone manufacturer can do this via a software update. The advice I can give you is to hope that with future software updates your phone manufacturer decides to include the secret menu (you can contact your phone manufacturer and request to add the SystemUI Tuner's secret menu).