Riot shield should be in ArmA. As a Military Simulation ArmA should have them. They should be very useful for example in CQB. There was a thread about Riot Shields 2013 a dev said that he will look into it... 2 years later still no Answer ( http://feedback.arma3.com/view.php?id=6156 )
There are only a handful bad made mods for it. I would like to see one from BIS that works fine.