One of the things that helped T-Mobile to grow so fast was the reintroduction of unlimited data plans. We all know that "unlimited" really isn't unlimited though and when the T-Mobile One plan debuted it had a cap of 32GB of use per month before data prioritization. That cap was raised to 50GB in September. Customers on that unlimited plan and many of T-Mobile's other plans had access to data in other countries baked in if they needed it.
That roaming data was specifically for Mexico and Canada. T-Mobile has now stepped in and is limiting the amount of roaming data available to people in those countries starting on November 12. T-Mobile wrote, "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)."
T-Mobile says that this change was made to "prevent usage beyond the intent of the product"; reading between the lines likely means that customers were costing the company too much money on the roaming networks.
As for why T-Mobile is making this change, it wrote "Mobile Without Borders is an incredible benefit and allows customers to stay connected when traveling in Canada and Mexico. In order to prevent usage beyond the intent of the product, we implemented a limit on the amount of monthly 4G LTE data. Less than 1% of people with this benefit travel to Mexico and Canada use over 5GB a month. 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)."
The good news here is that if you are traveling and eat up your LTE data allotment, you won't be charged for overages, your speed simply gets cut back to 128kbps or 256kbps. Granted that makes mobile data frustratingly slow to use and won't work well for streaming video. T-Mobile will send roaming users messages to let them know at 80% usage and at 100% usage. T-Mobile also notes that you will not be able to use your Data Stash in Canada or Mexico, but will accrue Data Stash data while in those countries.