It's there and works fine. It's characteristic type is 00000030-0000-1000-8000-0026BB765291.
It's absurd for this property to be deprecated as it's the only real way to tell accessories apart if the user has multiple iOS devices, and also the only way to remember which accessory the user was referring to later if they configure something in the app, especially since accessory UUIDs are totally unreliable changing frequently even from simple things like deleting an app and installing a TestFlight version of the app.
I think everyone is just assuming if they ever actually deprecate it, they will provide an equivalent replacement. All the hardware people are still required to set it, so it seems there is no reason whatsoever to remove it.
Maybe we should file RADAR bugs about this now so that it either doesn't get deprecated or is replaced with a proper solution that is not totally broken like accessory UUIDs.