Now some may call this fact others May call it fiction or opinion but when the iPhone 10s and 10s plus saw weak sales Apple decided to bring back the X. These were in limited supply and even if T-Mobile has it on their website that doesn't always mean they will get any more stock in. It's just like when Apple brought back the SE very temporarily not long ago.
The iPhone 11's aren't too far away either from being teased. It might be better to wait for the latest model.
Manage your T-Mobile account