It would probably be moved to Apple One, while not actually being lost from T-Mobile. i.e T-Mobile's option is just for payment, not content or access and is just part of your account with T-Mobile. If you ever cancel Apple One, you can return to it being included by T-Mobile.
At the end of the day, both options provide it, so hardly seems relevant from where its cost is getting covered.
You won't get a discount on Apple One if that's what you are thinking by having it stay with T-Mobile though even if it were possible. Apple One is a packaged deal for all included services.
The service itself is in all instances provided by Apple, the difference is just where it's price getting covered from.