This is not altogether surprising, of course, as Samsung extended its One UI public beta testing program to the GS8 and S8 Plus a little over a month ago. But the company initially expected to be able to bring Pie-flavored goodies to the two handsets in March, revising its schedule with a promise of earlier over-the-air delivery shortly after the first beta build was released.
Clearly, final tests went off without a hitch, although certain regions are still likely to have to wait until next month for official Android 9.0 rollouts on the Galaxy S8 and S8+. That obviously includes the US, both as far as unlocked and carrier-specific variants are concerned. After all, the US unlocked Galaxy S9 barely joined the party this week, while the Galaxy Note 8 is yet to be updated on a single local network.
So far, we've only been able to corroborate Android Pie release reports for the Galaxy S8 and S8 Plus in Germany with the help of Reddit comments and screenshots, but if history is any indication, a quick spread to more countries across Europe and Asia is probably in the cards. The stable OS makeover is already rolling out for everyday users (as in, not just beta testers), weighing in at a whopping 1.6GB or so.
Apart from Google-specific Android 9.0 features, add-ons, and UI tweaks like Adaptive Battery, Adaptive Brightness, and gesture-based navigation, you have loads of Samsung-proprietary One UI changes to look forward to, largely focusing on simplifying the user experience and improving one-handed interaction.
Don't forget to perform frequent manual checks for new updates on your Galaxy S8 or S8+ if you want to download and install this massive goodie pack as soon as possible by accessing your Settings menu, then "Software update", and finally hit "Download updates manually." If there's nothing there, just give it a few more hours and repeat the process.