Samsung doesn’t release more than two major Android OS updates for its devices and doesn’t really keep flagship devices on the monthly release schedule for security updates beyond three years. That’s just the way it is and there’s no indication of this changing any time soon.
With the Galaxy S8 and Galaxy S8+ now crossing that threshold, it’s no surprise that the company has shifted both of these devices to the quarterly cycle. That means both of these devices will no longer receive monthly security updates. Given that the Galaxy S8/S8+ hit shelves on April 21, 2017, it has been a full three years since these devices were released.