Android 9 brings support for gesture-based navigation but current phones can choose to retain the conventional navigation buttons. While OEMs making Android One devices are still free to choose whatever navigation UI they deem fit, Google says that future Pixel phones will only feature the new gesture-based UI.
https://www.notebookcheck.net/Android-Pie-gesture-navigation-will-be-a-mandatory-feature-on-the-Pixel-3.321216.0.html