I'm running Ubuntu 14.04 with OC 9.0.4, and recently I found out that the PHP version shipping with Ubuntu is version 5.5, which has has apparently reached EOL PHP supported versions.
As I'm obviously concerned about the continued stability, performance and security, what is the recommended course of action?
Distributions continue to maintain eol'd php versions. There is no immediate action required on your side.
Please note that with ownCloud 9.2 we are dropping support for php 5.4 and maybe 5.5.
Keep in mind that they probably can't backport every single bug or security fix to EOL versions. I think the chances for PHP 5.5 are better than for older version like PHP 5.3 or 5.4 but still something to consider.
ownCloud is twice as fast on PHP 7.0. If you can update your system, I would consider to use the more recent Ubuntu 16.04.
Ubuntu only provides 5 years LTS support for packages in main, e.g. universe is not updated any more. You can check your setup with: ubuntu-support-status
Thanks for your responses. I will definitely look into upgrading to 16.04; if not, I will still be supported for some time regarding PHP 5.5.