Surely fixable by the server checking for visibility of the shadow as well? I don’t know Valorant but I assume that could actually become a strategic element to not hide with a light source at your back.
Positioning is already a huge part of the game, and every wall, corner and box to duck behind has a strategic purpose that one must be aware of, just not based on lighting but geometry and game strategy. So adding a whole other mechanic to it wouldn't be interesting.