In Plesk Interface, some PHP versions are marked as 'outdated'. Why is so and what does it mean?
'Outdated' mark near some PHP versions was introduced in Plesk Onyx 17.8.
This mark means that the PHP version has reached its End-of-Life.
Versions of PHP marked as outdated are: 5.2.x, 5.3.x, 5.4.x, and 5.5.x. Refer PHP website for more details: https://php.net/supported-versions.php
These versions are still available for installation via Plesk Installer and can be enabled on a website on one's own risk.