After upgrading Plesk to Onyx 17.8 and updating the extension WordPress Toolkit to the version 3.0.0 or higher, the option WordPress > Global Settings > Enable integration with Addendio is missing so it cannot be disabled:
Also, this option is automatically enabled even if it is disabled prior to the upgrade.
In the WordPress Toolkit 3.0.0 or higher with the new UI (which is available only in Plesk Onyx 17.8), the plugin and themes management is fully integrated with Addendio.
The option Enable integration with Addendio can be returned to the Global Settings tab of the WordPress Toolkit by switching to its old UI.
Use the instructions from the article Is it possible to switch WordPress Toolkit to old interface in Plesk Onyx 17.8? to do that.
Also, an in-house solution for the plugin and theme management is being developed currently by Plesk development team (issue ID EXTWPTOOLK-1172). It should fully replace the integration with Addendio.
It is planned to be added to WordPress Toolkit in one of its future releases.