Is it possible to uninstall PHP by OS vendor on Plesk for Linux server?
Yes, PHP by OS vendor can be safely removed.
Go to Tools & Settings > Updates > Add/Remove Components > Web hosting > PHP interpreter versions.
Select PHP 5 from OS vendor for removal.
PHP by OS vendor is used by Plesk utilities, so the removal can break Plesk installation.
There will be a warning about it in the Plesk installer:
PLESK_WARN: The following components depend on the components you are going to remove or conflict with components you are going to install:
All language localization for Plesk, WordPress Toolkit, Advisor, Git, Skins and Color Schemes, Resource Controller (Cgroups), Webalizer, AWStats, Server Health Monitor, Watchdog system monitoring, SEO Toolkit, Plesk Migrator, Social Login, Domain Connect, Plesk Web Server Configuration Troubleshooter, Let's Encrypt, Plesk Firewall
These components have been selected for removal below. To confirm removal of all additional components, click Continue. To cancel removal or installation of certain components, select the corresponding option from the actions list.
The following components depend on the components you are going to remove or conflict with components you are going to install, but cannot be implcitly removed:
Either confirm that you allow removal of these components by selecting the corresponding option from the actions list or make sure other components that these depend on aren't removed or conflicting ones aren't installed, then click Continue. To abandon components removal click Cancel modifying.