Even if it doesn't, T-Mobile only wouldn't bother me (as a current T-Mobile user). The pricing is great and the international roaming works a lot better than T-Mobile's own slow but unlimited solution.
You can already do that AFAIK. I don't think you need anything special if you just want to use t-mo all the time.
Provider switching can be done directly at the SIM level; the phone just sees it as if you’ve swapped your SIM.
I’ve used such a SIM for ages on an iPhone and no issues what so ever so I expect them to use the same method.
Perhaps the iPhone XS could support switching via its new dual sim feature?