> 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.

They've got firewall (from US company Palo Alto Networks, btw), to block GTP traffic over GRX (which is used for roamers) based on their rules.

And unrestricted access is already unavailable in Tibet and Xinjiang.

I’d also love to have more details on this. I currently use Google Fi on my phone and laptop when in China, that’s the only way I’m able to be productive. If there are limitations on this or it’s a short term solution, that will affect my future plans in China. So any links or more details would be appreciated!

Do you have a cite for that? I'd like to read more.