Apple retires Snow Leopard from support, leaves 1 in 5 Macs vulnerable to attacks

The change was probably due to Apple's accelerated development and release schedule for OS X, which now promises annual upgrades. The shorter span between editions meant that unless Apple extended its support lifecycle, Lion would have fallen off the list about two years after its July 2011 launch.

None of this would be noteworthy if Apple, like Microsoft and a host of other major software vendors, clearly spelled out its support policies. But Apple doesn't, leaving users to guess about when their operating systems will fall off support.