The problem is T-Mobile is paranoid about Band 12 VoLTE w/ E911 calling. They are testing it and will certify it once they are satisfied that it works correctly. I have unlocked my boot loader and installed 6.0. I can tell you that it works with Wifi calling and Band 12 VoLTE, but I have not tried calling 911 (and I will not for obvious reasons). I do not know what the hold up is for T-Mobile certifying it, but that is the reason it is held up.
As for older devices, they can safely take 6.0 because they do not have the hardware to support Band 12. So there is no danger of having a Band 12 connection but not being able to dial 911.
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When you make kernel changes and radio changes it needs to go through the FCC and things also need to get re-certified across the board. Simply dialing a call isn't going to show if E911 works or not. The problem is hand off in areas that don't have band 12 and you are switching between them especially in fringe areas. If it doesn't hand off, you can't place a call even if another carrier has reception. (All carriers are open to E911 calls regardless if you are a subscriber or not, much like landlines. No service, you can still call 911). T-Mobile would be held liable even under the eyes of the FCC.
Also there is a zip file of the exact update, and also the factory image. (They are all the same. They are once again homogenized). If you flash those and something happens, then T-Mobile is off the hook for any liability.