To be honest, we just missed that, as the archive, was never a popular feature. I've noted down to fix this in 2.9.1 or 2.9.2. My take is to have a checkbox below the category selector to fetch items from children categories (or not), so there's a similarity in how we implement such category filters elsewhere (e.g. in the K2 Content module).

Thanks for bringing it up.

I suppose same-level categories. Noted. :)