The Riot Shield is a huge advantage to Assaults compared to other classes in terms of back protection.
It simply reduces the amount of shootable areas to a tiny portion of the top of the head, the legs, and some of the arms. The only way to shoot these people when they're prone tends to be running towards their legs and shooting them or proning and shooting them in the head. The shield needs to be made slightly smaller when undeployed.
Something like this, for example, where the sides and top pop out as well.
Otherwise, it literally is portable cover, especially when an assault gets killed and their shield simply becomes a shield on the ground that deflects all bullets fired in a prone position.
Do you want them to pull the shield out of their pocket? Leave the shields alone. I have no problem killing an assault from behind, just aim low.
That's actually not a bad idea, considering the assault rifles appear out of thin air as well. From a game balance perspective, I think it makes no sense that the class with the most health and armor also gets the benefit of being inpermeable to 90% of headshots and 100% of torso shots from the flank, when all other classes and even HEAT Assaults are vulnerable to this. Either the coverage for the head is reduced, or the shield should be made smaller when undeployed.
The shield even causes problems when the Assault is dead. The shield blocks bullets passively even when the Assault is downed, which has caused some issues with corners being completely unshootable.