Updates for Mavericks (OS X 10.9) and it's two predecessors Mountain Lion (10.8) and Lion (10.7) were released February 25th, along with an announcement that Apple will no longer be releasing updates for Snow Leopard.
Snow Leopard was also ignored in December when Apple updated Safari 6 and 7 in preparation for newer editions of OS X, Infoworld reported
With Snow Leopard's retirement, one in five Macs are not running operating systems that are compromised because of un-patched vulnerabilities. Many people have held onto the operating system because it was the last version of OS X able to run applications designed for the PowerPC Processor, the Apple/IBM/Motorola crafted CPU that Apple used before it switched to Intel in 2006.
Computerworld speculated that the change was probably due to Apple's accelerated development release of OS X, which promises annual upgrades. Because of this shortened time span between editions, there is a more extended life-cycle, causing Snow Leopard to be bumped off the priority list.
"Let's face it, Apple doesn't go out of their way to ensure users are aware when products are going end of life," said Andrew Storms, director of DevOps at security company CloudPassage, in a December interview.
Apple seems to have decided to continue to support Lion and Mountain Lion for awhile longer. The updates on the 25th patched 21 vulnerabilities in Lion and 26 in Mountain Lion.
"Apple might have also retired Snow Leopard since it's the last remaining operating system that supports 32-bit Macs, which contain first generation “Core” processors," reported
ReadWrite. "Mac OS X 10.7 Lion, and all versions that followed, are 64-bit. (Apple's newest iOS devices are also 64-bit.)"