T-Mobile is all in on unlimited data plans—you can't even buy a traditional capped plan anymore. Oh, there are still limits, of course, but Tmo isn't alone there. However, T-Mobile is making one of its unlimited limits a bit less irksome. Unlimited users now have to use more data before the network starts slowing them down.
Like other carriers, T-Mobile drops your network priority after you use a certain amount of data. As of early this year, that limit was 32GB, but now that's increasing to 50GB. This is not to be confused with the soft cap T-Mobile has on legacy limited data plans. When your capped data bucket runs dry, you are knocked down to 2G speeds. Unlimited users who hit the aforementioned limit are simply "deprioritized." You get slower speeds when the network is busy.
So, unlimited lines that use more than 50GB in a billing cycle will now trigger the prioritization point. Your mobile data will likely slow down when the network is congested during the day, but it might be fine at other times. It all depends on local network conditions. Most of T-Mobile's unlimited plans include video throttling, so you might have trouble reaching 50GB in a given month. AT&T and Verizon both use 22GB as the prioritization point, which you could more easily hit.