Previously, Android users only had a basic toggle to activate or deactivate spoken notifications sent to Assistant headphones for all apps at once. Now they can go app by app, switching on the vocalized alerts to their comfort. Messaging services are the only ones that deliver notifications by default, so you'll have to click the others 'on' manually. Finally, this doesn't seem like an upgrade to the Pixel Buds themselves but through the Google app, theorized 9to5Google, since they appeared after downloading Android 8.2.