If you're thinking more of a force-field type shield the Tesla Armour the Enclave employed in Fallout 3 and sometimes in New Vegas that resisted energy weapons as a special effect. That's kinda as far as it can go without putting in special mechanics for them.
If you're thinking full on Legionary scutum for Caesar's Legion then I think thats something they logically would have but never really shown as having. It doesnt make sense to bring a physical shield to a gun fight, but maybe in the tribal lands that the Legion conquered they made use of shields in massive formation combat with undisciplined tribal warriors. When you go up against the service rifle using NCR troops it turns into human wave tactics and you just need to charge as fast as you can with your machete, the shield becomes dead weight with bullets getting through it no problem.
If you're thinking riot or ballistic shield or something like that for Fallout 4 then it becomes something that would logically exist but isnt represented in game since it would be a dead end once you get power armour, so its much better to put development resources into other things. Would be cool to see it in a mod or something though, even though thats probably not going to happen with the GECK being the way it is.