Projectiles are a pure visual element. The simulation engine is agnostic of the actual arrow or stone. Ranged attacks are simply a velocity and the damage code just run a calculation, there is no physics simulation going on. The visual projectile is also a local entity, which can be considered to not be part of the game, is not serialized, does not affect the game state and is there purely to be seen by the local player.
Unlike FPS games where bullet physics are actually simulated, for RTS games with thousands of units, its not going to be feasible. Have to do ray casts over several entity meshes per turn per projectile. There is a potential optimization if only buildings are considered, which do not move, and are also reduced to cuboids. In which case, it might be potentially okay. However, projectile paths are not rays, there is some ballistics going on. A ray will not have a curved trajectory going over a wall. So I assume a more expensive form of collision detection needs to be done.
This will be a complete overhaul of how ranged attacks are currently implemented.