Turned on my Bluetooth headset, took a second two connect (Bluetooth Icon shows dots), and then took another couple of seconds for PulseAudio to recognize the device and then the OSD showed up.
If a device icon is available (e.g. headset) it is shown, otherwise the volume icon (using the correct icon depending on volume) shows up:
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* Verified that no OSD is shown on startup (the default changes once from none to whatever is the default)
* Verified that no OSD is shown when user explicitly changes the Default in the applet
* ~~Did not test what happens if I plug in headphone jack (where the profile on the same device changes from speaker to headphones)~~ Plugging in headphone jack also shows an OSD here, the device doesn't have a different icon, though, but could be that a newer PulseAudio version sets `formFactor` more often
* Changing output configuration in volume settings might spawn the OSD when you disable/enable a device that used to be a default at one point
There's a setting in the applet, default is enabled:
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