Sometimes these are left out.
Our 2003 Exped 5.4L has one.
The valves are usually installed to increase the efficiency of the A/C by stopping hot coolant from going thru the heater when in MAX A/C mode.
The heater core usually shares the box with the A/C evaporator.
With just a plastic wall between the two stopping the coolant flow will reduce the heat in the box for MAX A/C mode.
A mechanic can add one, but it does requires a vacuum signal from the heater control panel to activate the valve, so not that simple.