Cleaning Up Classes In Lists

Removing an item from a list only remove the reference to the item contained in the list. A list does not hold the object but just a reference to it.

As a result, when doing:

 list.RemoveAt(index);

the object is still in memory and could still be retrieved with GameObject.Find("Name").

T$$anonymous$$s below will remove the object on collision:

 void OnCollisionEnter(Collision col)
 {
    if(col.gameObject.tag == "Projectile"){
       projectileList.Remove(col.gameObject);
       Destroy(col.gameObject);
    }
 }

That considers you are storing your projectile in a list right after you instantiate them. T$$anonymous$$s will remove from the list and destroy the object. Later on garbage collector will free the memory when needed.