On 25/7/2012 Apple released Safari Version 6.0 for Mountain Lion (OS X 10.8) also available for Lion (OS X 10.7) but not for earlier software. At the same time, support for Windows seems to have been silently dropped, with the previous latest Safari version 5.1.7 no longer available for download and Safari excluded from Apple Software Update on Windows.
Given the significant number of security fixes in 6.0, use of earlier versions is no longer considered safe. If you aren't able to upgrade to 6.0, install and use Firefox, Chrome or Opera and uninstall Safari.
Those able to keep up with Apples' current hardware will find the upgrade welcome and exciting. Those who prefer to keep hardware for longer than about three years may find this decision difficult to understand.
Apple reveal virtually no information about their software and hardware policies, so observers are forced to guess.
The bottom line is that Apple seem unwilling to provide reasonable ongoing support for their products for more than about three years. That may seem an unreasonably short time to many people, much shorter than for example MS. The main risk is inadequate security in older software. Caveat emptor!