AT&T published a well-defined policy on the limits imposed on unlimited accounts (see "AT&T Message to the top 5%" - here).
As oppose the previous policy limiting the "top 5%" (something that can't be measured or verified by the subscriber), the policy (here) now is "Each time you use 3GB or more in a billing cycle, your data speeds will be reduced for the rest of that billing cycle and then go back to normal .. If you have a 4G LTE smartphone and still have an unlimited data plan,the same process applies at 5GB of data usage, instead of 3GB".
And AT&T will say this only once!: "You'll receive a text message when your usage approaches 3GB in one billing cycle" but "The next time you exceed that usage level, your speeds will be reduced without another text message reminder".
Why all that? because we sold too many smartphones - "Experts agree that our country is facing a serious wireless spectrum crunch" !