The OnePlus 8T was announced today, and T-Mobile is the only carrier that will sell it in the US. This time around, T-Mobile's model gets a "+" stuck to the end of the name. It turns out that the T-Mobile model will come with IP68 certification, something that the unlocked model does not boast.
IP certification against water and dust resistance is something that has long been missing from OnePlus phones, though the carrier models of the OnePlus 8 did sport the same IP68 certification that the unlocked model was missing as well. Why T-Mobile's 8T+ is certified when the unlocked 8T isn't is unclear, though it sounds like T-Mobile simply decided it was worth the certification costs to get the IP68 rating on its version of the phone. We suspect that the T-Mo 8T+ and the unlocked 8T are very nearly identical in terms of hardware and design, but OnePlus simply didn't want to get the 8T certified for whatever reason.
T-Mobile will sell the OnePlus 8T+ for $749.99, which is as near as makes no difference to OnePlus's own $749. That makes things even more puzzling, as one would imagine the IP-certified phone would cost more due to the certification costs. Perhaps T-Mo is absorbing said costs because it simply thinks an IP68 rating will help move more units.