How correctly classify the content items, including handling the distinction between published, draft, trashed, and unpublished states?

In what context are you wanting to work out the state?

I didn't understand your phrase, I use Umbraco and thanks for all who was part to build this excellent CMS! *Back to subject*: I use the

contentService

as below but trashed property I find it always false even if the item in Recycle Bin!

// Retrieve all content items
            var allContent = _contentService.GetRootContent().SelectMany(GetDescendantsRecursive).ToList();

var trashedCount = allContent.Count(c => c.Trashed);

I was referring to in a request, the UI etc? I had no context on how you can tell without some code

No problem, I think the idea clear now after the code above.

_contentService.GetRootContent() I think is why you arent seeing any Trashed items as that query will be removing the mas the "Root" id for the recycle bin is different if i remeber correctly

Excellent, I confirm it is working now. Thank you, @Matt Wise