Additional directives section is not visible for customers in Domains > example.com > PHP Settings
This is by design.
According to Plesk documentation such behavior is by design:
Note: Setting additional PHP directives is available to the administrator only and is never available to a customer. When the administrator sets additional PHP directives to a particular subscription, the subscription does not get the Locked status. Therefore, the added directives and other changes in PHP settings can be lost when the subscription is synchronized with the service plan.
Refer the article below: