But that doesn't mean my 2009 MacBook Pro is dead or will become immediately useless. Quite to the contrary, I expect to use my MBP securely for several more years.
Mac OS X El Capitan will continued to be supported by Apple with security patches for some amount of time. How long? My research shows that Apple supports a deprecated operating system for at least two years after the release of its subsequent operating system.
So if Sierra is released on October 1st, 2016, El Capitan will be supported by Apple until at least October 1st, 2018.
Then what? Well, by rights, most people should no longer use El Capitan as it becomes security-obsolete. But some more technically-minded people might have some options:
- Hack Sierra onto the machine. This may extend the security of the machine until Sierra is obsolete, perhaps around October 1st, 2019. No one can promise that Sierra will continue to operate properly on a 2009 MBP, but it could.
- Manually patch El Capitan. This may be reasonably possible depending on the security vulnerabilities revealed after obsolescence. For example, there is an NTP security patch that can be manually built and applied to the obsolete Lion.
- Install another operating system onto the machine, such as Linux or Windows.
- Use the machine in an off-line way. For example, the machine could potentially be used securely if it never shares data with the outside world.