Back in 2015 T-Mobile announced a new feature for their customers called Mobile Without Borders. This allowed you to travel to Mexico and Canada from the US without incurring any extra charges.
T-Mobile has now announced that they will be placing a cap on LTE data for customers traveling to Canada and Mexico of 5GB.
Starting November 12, 2017 customers on T-Mobile ONE, Simple Choice North America, or legacy rate plans with a qualifying feature will receive unlimited talk, text and data with a maximum of 5GB of 4G LTE. After 5GB of high-speed data is used in Mexico and Canada (or your high-speed data allotment is reached, whichever comes first), customers will stay connected with unlimited data at Simple Global speeds (up to 128kbps for most plans or 256kbps with T-Mobile ONE Plus).
This new cap comes into place next month and you can find out more details about it over at T-Mobile at the link below.