All coils are not A-coils. If coil is of suffecient capacity,it will cool just fine. Removeing/opening plate during heating operation would indeed increase air flow but who know's if that's nessary without specing furnace,ducts and evap coil..

Pic?

Doesn't make sense to me to do that. Also, the only way it would open up on heating is some sort of motorized damper.

I would get if you are on a multi level and want to adjust cooling to the different stories but that would be done by adjusting ductwork dampers, not some sheet metal blocking your A coil. Kind of surprised that your cooling worked and didn't freeze up.

i can't say if the coils ever froze as we just got the house and have not used the a/s much this year. but i will not worry as i removed the old furnace and installed a late model 90% unit ans made a removable blocking plate so at least in the winter i will get more air and not have to worry about the furnace not getting air flow to turn it off. thanks