Since introducing the "unused content elements" feature we need a way to allow third party code to decide used/unused status
The implementation of https://forge.typo3.org/issues/67884 unfortunately leaves no room for solutions which for one reason or the other, uses virtual column position (colPos) values. Every such colPos value is automatically considered invalid if it isn't part of the backend layout - but it is possible to add colPos selection values which are not in the backend layout. The result is that such solutions, including Flux, are negatively affected because all nested content is shown as "lost" or "unused".
It is necessary to add at least a basic way for third party code to weigh in on the decision whether a given record is unused or not. A signal can solve this perfectly.
Updated by Nicole Cordes about 3 years ago
How to reproduce / see the problem:
1) Install provided extension issue82213
2) Create a new page
3) Insert the content element "Container" on that page
4) Add another child content element to field "Content" and save record
After you go back to page view, you will see the child content as unused element although it is a child of your container element.