To understand what the deprecation of SCA's means, one has to understand what they are and why they are used. And to describe this, we need to start at the very beginning, the definition of a plugin which is split into two different calls to static methods.
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin()
\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin()
The naming of those methods is bit irritating as registerPlugin
does indicate that it needs to be called