Grandfathered Unlimited Data Plan is not unlimited...

ATT reserves the right to throttle your data speeds once you reach the threshold, BUT ONLY IF NETWORK CONGESTION REQUIRES IT. In all likly hood, that won't be necessary.

This has been happening for the last 5 years. The two differences are 1) now you only get data speeds reduced if network congestion requires it, in the past it happened whether there was congestion or not and 2) Now you are getting a notice, in the past there was no notice.

If you have never had your speeds reduced before, it is unlikely you will now.

To a certain extent you are "preaching to the choir."  Most of us are other subscribers and get no special treatment from ATT.  I'm grandfathered on their closed international unlimited plan which similarly gives me unlimited US data (together with all the rest of the countries)

ATT was pressured to change its interpretation from automatic throttling to as needed throttling because of the FCC.  If you migrate your phone to LTE from HSPA you'll get the cap moved to 5 gigs. I've only been throttled once but was upset when it happened.  My experience was that the initial level of throttling wasn''t awful, eg Google Maps, and music streaming still worked.

The big trap on unlimited (domestic or international) is domestic roaming on non-preferred partners.  My wife got in trouble in North Dakota doing a mandatory upgrade to her GPS software (from an ATT partner) because the download exceeded 200 megs on her unlimited plan.  There is an exception to unlimited on these non-partner networks.  There aren't many of them anymore, but when you are on one watch out.