Read/Unread item
Please add "Read/Unread" item to Zotero like another Article Manager (Mendeley, Endnote). It's not reasonable to do it by tagging or folder creating.
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read
read, partially
skimmed
read; read again later
to read
I am with OP on that. There should be a functionality that unread items are moved or copied to a special collection, e.g. "Unread". They should disappear when an item is marked read. This is reasonable and that is why was implemented in other applications.
A saved search for the relevant tag will add/remove to a collection automatically on assignment/unassignment.
(You can assign regular tags to multiple items by dragging them to the tag in the selector on the left, but unassignment isn't possible in mass. It's also a lot less elegant than a single keystroke).