If you run an older version of OS X, does Apple supply security updates? How far back?

I run Mountain Lion (OS X 10.8.5) on one of my systems. For the longest time, whenever I click "Software Update", it says that there are no updates (though it does offer OS X El Capitan as an upgrade).

I can understand that Apple or any company doesn't want to support old software indefinitely, but if security updates are available as separate packages--without having to do a major OS upgrade--then you'd think that "Software Update" should offer it!