Thanks @Vitaliy ! As I understood from this topic, the main reason for using PHP 5.6 is its potentially greater security compared to previous releases of PHP. In tech support I was not hear any reason to stay on an outdated version of PHP. In my opinion this is due to the unofficial “motto” of system administrators “It works? Don’t touch it!”.
Through MAMP I conducted an experiment on my test machine. In MAMP you can change PHP version for the test domain on the fly. And I installed OJS under PHP 5.6. After that in MAMP I changed PHP version to 5.5. Everything works (it seems). However, it is possible that some parts of OJS may not work correctly. And I’m worried about this.