Samsung's new One UI isn't just a development in the company's previous design aesthetic, it's also noteworthy for being the vehicle for the Galaxy S9, S9+, and Note9's update to Android 9 Pie. You see, we've seen the early tests for One UI before, we know it's based on Android Pie, and according to Samsung's Developer Conference keynote, One UI is expected to land next January. Ipso facto, those phones should be getting Pie then.
Samsung may have been less than explicit in its announcement (or if they were, we didn't catch it), but all of the facts point to a January update to Pie via One UI. Even the One UI page on Samsung's site mentions Android 9 Pie in a footnote — though, again, it's less than explicit in revealing that's what it's based on.
Earlier betas of the One UI have leaked for both the Note9 and S9/S9+, and with them being based on Pie, and Samsung stating that the final release for this new UI is expected in January, we're confident Pie will be making its way to the phones then in some capacity — though historically not every model gets timely updates. Either way, set your calendars.