It used to be that all iPhones bought directly from Apple in the US came unlocked, meaning that you could use them with any carrier – even if you bought the phone with financing from another carrier. Now that’s no longer the case.
Apple has updated its FAQ and it now says that iPhones bought with carrier financing are locked to that carrier until the phone is paid off. Once you complete the final payment, the phone will be unlocked and you will be free to switch to a different carrier.

The updated FAQ states that you no longer get an unlocked phone with carrier financing
This applies to AT&T, T-Mobile and Verizon, which are the three carriers that Apple works with. There are two more options – Apple Card Monthly Installments and Apple iPhone Payments. These require activation on one of the three carriers, though as far as we can tell, this doesn’t lock the phone to that carrier.
Confusingly, the “See details” popups for T-Mobile and Verizon still claim that you will receive an unlocked iPhone, but maybe Apple forgot to update that text. AT&T’s popup doesn’t promise an unlocked phone.
Contradictory details for T-Mobile’s and Verizon’s device payment programs
Previously, people were able to get better deals by picking a favorable carrier financing plan and connecting the phone to another carrier either as the main SIM or as the second SIM. Now there is a lot less wiggle room – you either live with the SIM lock or you go through Apple’s financing plans.
PS. A possibly related and certainly coincidental change is that Apple no longer offers Boost Mobile carrier deals.