How important is that metal flap/disk on the gas filler neck cause it's missing on my 4runner?

 

If your talking about the one in the gas filler neck that whole idea started back when you could still find leaded gas.

The nozzles for unleaded where made smaller and those flaps were made to take the smaller nozzle and not allow the larger nozzle into the filler neck

Not sure why yours might be missing unless someone had a old nozzle on a tank at the ranch.

I don`t know that it would cause any problems.