As spotted by XDA, the pages have been updated only in the United States, though other markets are expected to follow. There is no mention of a launch date, but if last year's Oreo beta program for the Galaxy S8 and S8+ is anything to go by, the new one be launched sometime in November.
Owners of the US unlocked versions of the Galaxy S9 and S9+ can sign up for the Android Pie beta program through the Samsung Plus app. Once the launch date rolls around, you'll get a notification to sign up for the beta, but you'd better act fast, as there will be around 10,000 spots available.
Android 9.0 Pie on the Galaxy S9 and S9+ comes with the Samsung Experience 10 built on top. It introduces a refreshed user interface with a redesigned icon set and navigation buttons, darker backgrounds and notifications with rounded corners that closely match those of Samsung's Infinity Displays. Samsung Experience 10 also introduces a customizable, gesture-based navigation and a "lift to wake" feature.