The function was spotted by XDA-Developers in the form of an AOSP commit titled ‘Implement Bluetooth device volume memory.’
“Bluetooth volume memory is now handled on a per-device basis. The volume is stored in a shared preference and is loaded every time the device is active. Volume is persisted for devices that both do and don’t support absolute volume,” read a description on the commit.
The feature hasn’t been implemented just yet, but it would be a godsend for those who pair their phone with multiple devices. Right now, many phones have only one master volume level for Bluetooth devices, forcing users to manually adjust volume depending on which accessory is connected. So if you like to rock out in the car but don’t want to be defeaned by your Bluetooth headset, you could soon have individual volume levels for each situation.
You don’t have to wait for Android P to deliver this feature, though, as Bluetooth Volume Control is one notable alternative available for free at the Play Store. Still, a polished first-party option would be more than welcome.