While we are all guessing, here is my guess:
ATT is abandoning the 830. They are doing this so that they do not have to test Win 8.1.2 for the 830. 8.1.2 is required for the "kill switch" capability for any phone sold after July 2015. If ATT isn't selling the 830, then they don't have to test 8.1.2 for any new phones sold AND THEY DON'T HAVE TO UPGRADE THEIR CURRENT FLEET. Win10 is right around the corner and if someone even mentioned Win11, then ATT would probably dodge ALL Win10 phones saying that they will fully deploy Win11 phones but secretly hoping that someone else mentions Win12.
So, my guess is that the ATT 830 will not get upgraded to anything other than what it currently has. It's a Win-Win-Win for ATT. They don't have to test a device, they don't have to upgrade a device and they can push off deploying any new flagships (affordable or otherwise - does anyone else get a little chuckle when they think "affordable flagship"?) Windows Phones for another generation.
The moral of the story, DO NOT BUY A CARRIER BRANDED WINDOWS PHONE! It might sound good and it is for other phone OS's but it is a loser for Windows Phone. Plus, you will eliminate one of the foot-draggers in the OS update food-chain. Then you can just beat on MS for not deploying a phone. Seriously, do not get a carrier branded WINDOWS PHONE. If you want carrier branding, then go iOS!