I don't think Shield is going to be added as a spell. Reactions are automated in BG3, which they kind of have to be. Imagine what it might look like if they weren't. Every time an enemy moves past your character does it just pause and pass you priority to decide if you want to attack or not, or shield, or absorb elements, etc? It would start to really fuck up the flow of combat in a bad way if the game had to pause to confirm every reaction trigger from the player. That's why you automatically take reaction attacks, and why Shield is very unlikely to be added among other reaction spells or abilities.
