Hello,
other than in Shopware 5 in Shopware 6 the base classes of a plugin are no longer an event subscriber automatically. You will have to add an own subscriber class and add it to your services.xml file, if you want to subscribe to an event. Please also refer to our documentation for further reads https://docs.shopware.com/en/shopware-platform-dev-en/internals/plugins/plugin-quick-start?category=shopware-platform-dev-en/internals/plugins
Best regards from Schöppingen
Michael Telgmann