Having thought about this feature for roughly two years, I still believe it would be an organic extension in our community.
Let’s shape the metaphor using Nextcloud patterns: A category (or area, as we call them in our instance) should be a home folder for a special group, enabling otherwise disallowed users to share information with that group. Shared messages should be visually marked as a special case: some other groups or users are allowed to share information with that group (for example our school principal). For shared topics, group messages are the established pattern in Discourse.
But having them as a second-class citizen hides shared information from the users (in our case), since most of them are not aware of this special location at all. Showing them alongside the messages they are used to reading would be the ideal location in the user interface.
As wished here, a special “category” for “own stuff and stuff shared with me” as a custom category would extend this pattern to “find every information in the category view”.
I hope, helped by an LLM, I will be able to implement at least a mockup to test this scenario.