I've heard this is a temporary thing to make it easier for visitors, and it will stop working if you stay too long.

It is, but my understanding is that's because roaming is expensive for your home carrier, so they'll cancel your account due to unprofitability if you spend too much time roaming with it.

See the fine print in this T-Mobile document:

https://support.t-mobile.com/docs/DOC-3226

Not for extended international use; you must reside in the U.S. and primary usage must occur on our U.S. network. Device must register on our U.S. network before international use.

...

Network Management: Service may be slowed, suspended, terminated, or restricted for misuse, abnormal use, interference with our network or ability to provide quality service to other users, or significant roaming.