With many connectivity options on our smartphones nowadays, it’s easy to see how things can get very crowded on your status bar. LTE, WiFi, Bluetooth, and NFC get added to the time, battery level (and percentage), alarms, and maybe DND or hotspot icons to create a super cluttered status bar. Android has been allowing us to customize this in System UI Tuner for a while, and it has also made a few choices like removing the NFC icon when it’s not actively transmitting data, even when NFC is on. The same is happening for the Bluetooth icon in Android P.
On P, you can leave your Bluetooth turned on without it causing an icon to show permanently in the status bar. Only when it’s actively connected to a device will it pop up there. I can understand the reasoning behind this: 1. with notches, some displays won’t have a lot of space to show icons so every saved pixel counts and 2. users might turn Bluetooth off just to see the icon go away, which may make discoverability of new devices, especially smart ones, more difficult. As someone who always leaves Bluetooth on to connect to my smartwatch and wearables, and who doesn’t care to see the icon at all, I’m glad hiding it is the default behavior in P.