Samsung's Galaxy S8 and Galaxy S8 Plus are not even on store shelves yet in some territories, but the first update for the Galaxy S8 Plus, containing the April security patch, is already rolling out across Europe. The security patch appears to be the only thing in this update package. The patch contains a grand total of 49 security fixes from Google, as well as 16 Galaxy-specific fixes from Samsung, though the patch notes don't say exactly what those Galaxy-specific fixes are. The update isn't a terribly large download, and will bring the build number of Galaxy S8+ units that take it up to G955FXXU1AQDD.
While Samsung may not be revealing exactly what's in this update as far as Galaxy-specific fixes, those who want to know what's been patched up on Google's side are free to look over the update notes about April's patch right at the source, being Google themselves. According to them, there are no less than six critical level fixes in this patch, as well as fixes for privilege escalation bugs in essential Android core services like SurfaceFlinger, AudioServer, and CameraBase. Of the 49 total fixes to be found, a whopping 15 of the security fixes are either high or critical level. Some of the critical fixes are for bugs that were reported to Google as far back as December of 2016. To put it bluntly, this is a very important security patch.
The patch is rolling out in stages across Europe. Reportedly, users in Belgium and Germany are already getting the update, meaning that it should be rolling out across all of Europe in the next few days or weeks at most. Given the extremely slight differences between the Galaxy S8+ and its smaller sibling, it's surprising that there's no word yet on when the vanilla Galaxy S8 will see this patch in Europe. That can be said doubly so because this is a mere security patch; not a lot is changing, functionally. The two phones share the same software, so even if there was some sort of user-facing or deep-level change, it could theoretically be implemented just as easily on the S8 as on the S8 Plus.