Samsung's Galaxy S8 and S8+ devices won't be getting any more updates going forward — owners have to make do with the current state of their software if they plan to keep using theirs. As noticed by DroidLife, the old flagship devices are no longer included in the list of models currently getting security updates. The Galaxy S8 Active and S8 Lite are still in the list for now, but it's worth noting that they were released months later than the main S8 flagship phones.
It doesn't come as a surprise that Samsung has decided to stop rolling out updates for the devices. They were a hit when they first came out and provided Samsung with the best pre-order numbers it has ever seen at the time. However, it's been over four years since the phones were released — even Google doesn't support its Pixel devices for that long. The S8 phones got monthly updates for three of those, but Samsung pretty much gave everybody a warning that it'll stop supporting the devices soon when it put them in the quarterly security update schedule over the past year.
At this point, the oldest Samsung flagships still getting monthly security updates are the Galaxy S10. The Galaxy S9 and S9+ phones are in the quarterly security update schedule and will likely be receiving updates for one more year before the tech giant stops supporting them, as well.