As with the unlocked Samsung phones, the unlocked Galaxy S9 and Galaxy S9 + are the last member of the party. The Android Pie Update is not yet available as a stable build on both phones, even though all Carrier models have been updated.
As mentioned above, this is not a new topic, but it's worth it for those who do so. Continue to choose the unlocked versions over the carrier variants. Look, back to the unlocked Galaxy S7 and Galaxy S7 Edge. Samsung has proven with each new, unlocked version that they are in no hurry to update them. I have written several articles about it. I wrote that Samsung "hates" their unlocked cell phones, they generally did not update like the Carrier variants and threw a handful of "finally" takes when they get major operating system upgrades. The Marshmallow S7 edge update on Nougat took place 2-3 months later than the carrier models. With the Galaxy S8 and Galaxy S8 + the waiting time from Nougat to Oreo has been shortened to a few weeks. We were able to achieve a similar rollout with the Galaxy S9 and S9 +. All S9 carrier models are upgraded from Oreo to Android Pie and Samsung's One interface, so we're just waiting for activation.
It's hard to know why Samsung recently updated its unlocked phones. These are the phones that are always allowed to participate in beta builds. My unlocked Galaxy S9 has been running One UI and Android Pie since November of last year, but is still in a beta phase as the carrier models that were not part of the test have stable pie. The same happened with the Galaxy S8.
A moderator of the Samsung Forum once said that unlocking models may be more difficult to update than support variants because they undergo rigorous testing for all supported networks. That makes sense, unless they are always part of the beta, as if it would be easier to work with. How do you explain that?
In the end, we'll likely see Android Pie and One UI on the US Galaxy S9 and Galaxy S9 + in the next few weeks or days. I can not imagine that it will take much longer. However, if you're a phone owner unlocked by Samsung because you like freedom and flexibility, you should know that you may not always pay for the software.