Unity How to handle objects going off screen

Create an empty game object with a (2d) box collider. Scale and position it in your scene so it catches bullets or everything that can fall in it. This makes sure GameObjects get destroyed and don't linger around.

DeathZone.cs
using UnityEngine;
using System.Collections;
 
public class DeathZone : MonoBehaviour {
 
	// Use this for initialization
	void Start () {
 
	}
 
	// Update is called once per frame
	void Update () {
 
	}
 
	void OnCollisionEnter2D(Collision2D coll) {			
		Destroy(coll.gameObject);
	}
}
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