In short: A build of Android 9 Pie with Samsung's latest version of its "Experience" skin, version 10, has shown up on a Galaxy S9 Plus. The firmware was posted and a user was able to install it on their Galaxy S9 Plus. Which is able to give all of us a good look at what Samsung is cooking up for its Android Pie update, that should start rolling out soon. There are quite a few changes in this version of Samsung Experience, including a new Dark AMOLED theme for the device. This uses actual black colors, and not dark gray like many apps and even Google itself, allowing the user to get even more battery life out of the device. The toggles in the Quick Settings are also very similar to what you would find on the Pixel 2 series. All of the system apps are also getting black themes here, which is a really nice look from Samsung.
The settings has also changed a bit. It's more of a black border with white settings here, which does look a bit odd in the screenshots, but it's important to remember that this is an early build, and it will likely get straightened out before the update rolls out. Samsung is also bringing native gesture support in the Experience 10 update. Where you can swipe up for back, home or recents. There isn't a different gesture for each one, like on other smartphones, instead you can swipe up on the left to go back, swipe up in the center to go home and swipe up from the right to to recents - or you can swap recents and back, like you can with the software buttons right now.
Background: Many were upset that Samsung launched the Galaxy Note 9 without Android Pie, but seeing as Android Pie had launched just a few days before, it was not a huge surprise. Now seeing all of the changes that Samsung is making with its Android Pie update, it's pretty clear why the Galaxy Note 9 did not have it launch. This is a pretty big update, and definitely warranted.
Impact: This update likely won't be coming to any Samsung devices (officially) anytime soon. It appears to be a pretty big update for Samsung, and it will likely need plenty of testing. The good thing here is that the firmware is available for those that do want to download it and install it on their Samsung device.