All Android 9 Pie devices will be able to "Turn on WiFi Automatically," i.e. utilize a quality-of-life feature Google first introduced with Android 8.0 Oreo last year. As evidenced by a number of source code snippets discovered by XDA Developers, the flag that previously allowed original equipment manufacturers to enable and disable the functionality in question has been removed from Android 9 Pie entirely as the service is now supported by default. The development marks the full implementation of yet another feature that Google added to its mobile ecosystem in a gradual manner, first introducing it as an optional functionality in one new edition of Android and then making it an integral part of the package in the following build.
Project Treble also followed a similar deployment cycle, having launched as an optional part of Android 8.0 Oreo (at least for devices being upgraded to the newer OS) before becoming mandatory in Android 9 Pie. The ability to automate Wi-Fi connectivity on your Android device has been selectively available on Android Oreo, with Nexus-series handsets and a wide variety of third-party electronics failing to adopt it. In practice, the feature allows you to have your smartphone or tablet automatically turn on its Wi-Fi module when it's in range of a saved network so as to intuitively switch to a better connection and save help you save mobile data in the process of doing so. Devices that support the service on Oreo have it located in the "Network & Internet" menu of the system Settings app, hidden under "Wi-Fi Preferences" of the "Wi-Fi" section.
The tool itself still behaves in the same way on Pie as it did on Oreo, meaning it also requires you to have "Wi-Fi Scanning" turned on under the "Location" menu of the Settings app. Android 9 Pie is presently only available on Google's Pixel devices and the Essential PH-1, though it will start hitting a broader range of handsets this fall.