Apple tend to continue supporting older versions of macOS for about three years (usually when it's close to being three versions behind), but this isn't guaranteed because Apple do not have a minimum support period for macOS releases unlike Microsoft with Windows. With Apple software, it's an unfortunate guessing game.
To give you some context, OS X El Capitan (this is the release prior to macOS Sierra) was last given an operating system security update in March 2018 but Safari for OS X El Capitan last received a security update last month in April 2018. As we're approaching the middle part of 2018, we may not see another security update for OS X El Capitan (again, we are nearing the presumed three year support window). That said, macOS Sierra is only one release behind the current and so it should continue to receive security updates until next year. This isn't guaranteed.
The only thing you can go by to see whether or not Apple may have discontinued support for an older release is to see what operating system versions Apple appear to be supporting when they are issuing security updates. You can see this information here:
https://support.apple.com/HT201222. However, it's important to remember that not all security updates will affect every recent version of macOS, which makes it even more difficult to know whether Apple have quietly dropped support for an older release.
All I can say with certainty at this point is that they have certainly dropped support for OS X Yosemite (released in October 2014), and that's because we haven't seen any security updates for OS X Yosemite since September 2017.
That said, macOS High Sierra has been out for a while now and most people find it far more stable and bug-free than Sierra, and you will find it performs faster on your Mac with Retina display because the Window Server now runs on top of Metal 2 (e.g. Mission Control animations are finally super smooth). If you decide to upgrade, make sure you backup your Mac first with Time Machine in case you need to downgrade. With every major operating system release, some people prefer to put the macOS image onto a USB flash drive and perform a clean install, which is also why some folks will wait a year or two before upgrading from one release to another. You shouldn't necessarily need to perform a clean install but upgrading in-place can, in rarer cases, cause issues that are not present when installed from scratch.