Apple's newly-announced macOS Sierra has a bunch of spiffy new features, but not all Mac users will be able to take advantage of them.
In fact, some won't be able to install the new operating system.
The last several versions of OS X (now macOS) have all supported Macs as far back as 2007. That's for iMac and MacBook Pro, while others needed to be more recent.
That said, not all features of recent operating systems (eg AirDrop and screen sharing to an Apple TV) were available on older hardware.
This time, Apple's drawing a fresh line in the sand and several older but supported models won't work with Sierra.
The minimum configurations for macOS Sierra are the late 2009 MacBook and iMac, and the 2010 MacBook Air, MacBook Pro, Mac mini and Mac Pro.
Reasons for dropping support for older models are varied. Some will suspect forced obsolescence, but that might just be a different way of looking at a decision that the absence of certain hardware capabilities (eg, Bluetooth LE) would detract too much from the overall user experience of the new operating system.
And it must be said that the performance of older models — especially those that had not been upgraded with additional RAM and an SSD — under recent operating systems left something to be desired.
Owners of Macs that can't be upgraded past El Capitan don't need to panic. Apple's position on security updates has never (to this writer's knowledge) been formally stated, but the company's practice in recent years has been to provide them for the current and two previous versions of the operating system.
So there almost certainly will be an supported OS for affected Macs for at least another two years.