We knew it was eventually coming, but it would appear that the Pixel 2 phones have yet another surprise hidden in them. Right now, supported Bluetooth devices on the Pixel 2 and 2XL are able to report and display their battery level. Unfortunately, checking that Bluetooth battery level on both the new phones isn't quite as easy as glancing at an ongoing notification, but there are a few different ways you can see it.
You're currently able to see the reported power level for Bluetooth devices in the submenu for the Bluetooth quick settings tile, on the "Paired devices" section of the Bluetooth settings pane, and in the new "Device details" section for Bluetooth devices. While there isn't a convenient ongoing notification or anything, at least there is a way.
Artem tried the Jaybird X1, X2, X3, and Run, and all of them were able to report battery level over Bluetooth. So not only has this feature been added to AOSP (as we all knew), but it's working with multiple different headphones and live right now on the Pixel 2 and Pixel 2 XL.
For comparison, the Google Store listing for the Pixel Buds shows off what an ongoing notification for the Bluetooth battery level might look like, though nothing seems to be triggering it right now. It could be this additional functionality hasn't been enabled quite yet, or maybe the notification pictured is only meant to work with the Pixel Buds.
All this should make it a bit easier to keep track of how much juice your Bluetooth headphones have left. Especially now that we're being forced away from using a headphone jack.