$cop not working in /s, probably /w as well. I've seen it not working sometimes in the main chat too.
I've typed /s $cop here:

Happens since the last update.
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Well, $cop totally gives you the player that you wanna target id, There will be a new commands regarding this will be added, But for now $cop will show the player ID though.
New command has been added to use, id and name will appear together though