Great write up and having just gone through this I figured I will throw in my 2 cents.
I have never run a Traxxas battery in any of my Traxxas cars. Nothing wrong with the batteries, they are just expensive.
Without a Traxxas branded battery all the telemetry works. The telemetry goes through the receiver to your transmitter which bridges it to Bluetooth and back to your phone. The extra wire is only for balance charging. If you have a Traxxas battery you might be able to still get the "balance adapter" to use it in a non-Traxxas charger.
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The Spektrum IC3 has a single wire between the barrels that is used with Spektrum chargers. I do not know about telemetry features of Spektrum so it may be used but I suspect it works similar to Traxxas as a battery is just a cells and wires (Except for the fancy Spektrum batteries)
I have switched everything to XT60 for two reasons. The first is it's seems to be gaining traction over the others and not just with RC stuff. The second reason is it stays together on the hard hits where I've had Deans come disconnect.
The internals of XT60 and IC3/EC3 are still a simple barrel and plug.