Samsung's Galaxy S6 family consisting of the original Galaxy S6, Galaxy S6 Edge, Galaxy S6 Edge Plus, and the rugged Galaxy S6 Active, officially reached its end of life and will no longer receive any sort of updates from Samsung, security or otherwise. All said and done, the phones will be ending their lives running Android 7.0 Nougat with the February 5, 2018 Android Security Patch on board. This means that those using the Galaxy S6 or a related model can still run the latest Android apps for some time, thanks to the phones' 64-bit processors, and there isn't much to worry about in the way of security at the moment, since many of the most major security scares in the last few years have been patched up as of February's update.
The end-of-life status means that users won't be able to get any further than Nougat from official software, but future security updates and even further Android updates may be obtained through community development. The vanilla Galaxy S6, for example, has ports of Android 8.1 Oreo available, though you have to be willing and able to go through the hassle and risk of unlocking your phone's bootloader and flashing a custom ROM. The entire Galaxy S6 family packs fairly powerful 64-bit processors and 3GB of RAM, so don't expect the community development train to stop for these popular devices anytime soon. Those processors do, however, have their limitations; they'll run Google Daydream content in a less-than-ideal fashion due to a lack of power for such things, and they completely lack the on-board machine learning and x86 instruction set emulation found in the Qualcomm Snapdragon 835 and many comparable processors.
All things considered, it's difficult to say that the Galaxy S6 had a bad run or was an unloved device; its premium design marked a turning point for Samsung, with its preceding flagships — the Galaxy S5 and Galaxy Note 3 — having used plastic builds that many customers complained feel cheap. The Galaxy S6 is not a bad phone to this day; it's well-designed, about as capable as most modern mid-rangers, moderately priced, and has a great screen. As a bonus, it's young enough that most used units floating around still have good batteries but old enough that its spare parts are now affordable and easy tutorials for replacing a bad battery are easy to find. If you happen to be using an older phone or need a backup and you have around $150 to spend, or less depending on where you look, the Galaxy S6 is not a bad place to start your search, running roughly equal to the LG G4, Google Nexus 6P, and the HTC One M9 in terms of raw hardware power and capabilities.
UPDATE: The Galaxy S6 Edge Plus and S6 Active are back on the list.