What is the lowest version of WordPress that is supported by WordPress Toolkit in Plesk?
WordPress Toolkit supports WordPress 3.6 or newer. It will detect older versions, but will mark them as outdated:
WordPress versions earlier than 3.6 were detected on your server. If they were installed through Plesk Application catalog, you can upgrade them by means of WordPress Toolkit in Plesk. If they were installed manually, you will need to upgrade them through WordPress itself.
Any website that is using a version of WordPress older than 3.6 will need to be updated before you can manage it with the WordPress Toolkit.