Earlier quoted context omitted.
T-Mobile is flexible and allows using any compatible LTE handset. They don't care what OS you use on it.
T-Mobile works nowhere, and is the only carrier which is actively countering net neutrality. Verizon is also required to allow using any compatible LTE handset, and they aren't allowed to care what OS you use on it. The problem is that nobody is developing them.
It works way better than a few years ago, and for the reference, almost all mobile providers, including Verizon[1] and AT&T[2] (not sure about Sprint) are violating Net Neutrality by giving preferential treatment to their own services.
[1]. https://www.dslreports.com/shownews/Verizon-Begins-Zero-Rati...
[2]. https://www.dslreports.com/shownews/ATT-Not-Worried-About-Re...