It recently occurred to us that deleting a collection (and its contents) that refers to or contains questions that other dashboards depend on will break those dependent dashboards. this makes sense to reference rather than copy the question, however it's not easily understood by the user that there is a dependency that may be broken. I think of this like a SQL foreign key constraint that does not allow you do delete keys that other tables depend on to enforce referential integrity. I expect this can be done by the Metabase internal database with a simple foreign key at the DB level and caught in the UI to show the user that they can't delete those question that other collections/dashboards depends on.