From AT&T’s support page regarding data usage on legacy unlimited data plans:
As a result of the AT&T network management process, customers on a 3G or 4G smartphone with an unlimited data plan who have exceeded 3 gigabytes of data in a billing period may experience reduced speeds when using data services at times and in areas that are experiencing network congestion. Customers on a 4G LTE smartphone will experience reduced speeds once their usage in a billing cycle exceeds 5 gigabytes of data. All such customers can still use unlimited data without incurring overage charges, and their speeds will be restored with the start of the next billing cycle.
I would assume that AT&T reserves the right to throttle any of their users if they’re experiencing network congestion. That’s just how networks work. I’m one of the customers that’s still holding on to my unlimited data plan, and I don’t plan on letting go of it anytime soon. I take comfort knowing that I won’t be charged extra if I happen to use more than 5GB of data in a single billing cycle.