Is the Deputy Card an Item?
No, it does not count as an item
The Rules, page 16 note on the middle of the right hand column:

Important: When adding up items before discarding
them, count all Common Items, Unique Items, and
Spells. In addition, the Deputy’s Revolver and the Patrol
Wagon both count as items. Allies, Skills, and other
cards do not count towards this total.

Since &quot;Deputy of Arkham&quot; falls under &quot;other cards&quot;, it does not count when adding up items.