One of T-Mobile's big "uncarrier" moves was to offer international roaming data to most customers for free. However, Canada and Mexico were special cases where you'd get full-speed LTE data on most plans. There wasn't any special limit on that data, which was remarkably generous—too generous, it would seem. T-Mobile is rolling that feature back and limiting LTE to 5GB per month in those countries.
Starting on November 12th, T-Mobile is altering its "Mobile without Borders" program for all customers on One, Simple Choice, and other legacy plans with roaming. Instead of having unrestricted LTE data in Mexico and Canada, you only get a maximum of 5GB. For those with old capped plans of 5GB or less, there's no change to the limit. For anyone on a One or classic unlimited plan, it's much more restrictive.
After you hit the 5GB cap (or less for some legacy plans), you will continue to get unlimited data; it'll just be very, very slow data. Most plans are throttled to 128Kbps after the limit, but One Plus receives 256Kbps after the cap. If you're still on a plan with Data Stash, that's not usable in Canada and Mexico anymore. This move isn't surprising when you consider T-Mobile is pushing unlimited plans more aggressively than it was in 2015.
Again, One plans still have unlimited LTE data in the US. It's just LTE in Canada and Mexico that's capped. T-Mobile notes you can still pay for the One Plus International add-on and get unlimited LTE in Mexico and Canada.