I think that refers to this PR (https://github.com/appium/appium-uiautomator2-server/pull/11117)
You can explore it by using the tests they implemented, such as https://github.com/vmaxim/appium-uiautomator2-server/blob/master/app/src/test/java/io/appium/uiautomator2/model/NotificationListenerTests.java11
The settings enableNotificationListener is in documentation in here: http://appium.io/docs/en/advanced-concepts/settings/#supported-settings11