2D Physics and Jump-Through Platforms

Physics2D has IgnoreLayerCollision, which is not the best solution but it could work if you switch objects to different layers based on whether player is above or below the platform

This is a simple script that I came up with, without using complicated scripting.

 //This script is attached to the player
         var jumpPower : float; //assign an integer for jump power
         var jumpKey : KeyCode; //assign jump button here
         var downKey : KeyCode; //assign down button here
         
         var box : BoxCollider2D; //assign the BoxCollider2D of your player and set it to is trigger
         
     function Update () 
     {
         //Jump
         if(Input.GetKeyDown(jumpKey))
         {
             rigidbody2D.velocity.y = jumpPower;
         }        
         
         // If you want your object to go down on your platform
         // Similar to the Contra game platform feature.
         if(Input.GetKeyDown(downKey))
         {
             box.isTrigger = true;
         }
     }        
 
 
         // have a seperated gameobject that has a boxcollider2d make sure is trigger is checked
         // in my case I named it Trigger and parent it to the platform
         // and make sure it is just a little under your platform.
         // your platform should have a boxcollider2D is trigger off
         
         // detects trigger and makes your players boxcollider2D isTrigger true which disables
         // collision and make it pass through your platform
         
         function OnTriggerEnter2D (hitInfo: Collider2D)
         {
             if(hitInfo.name == "Trigger") 
             {
                 box.isTrigger = true;
             }
         }
         
         
         // Exits your Trigger and set your player Collider2D.isTrigger false, which makes it collide
         // to your platform again.
         
         function OnTriggerExit2D (hitInfo: Collider2D)
         {
             if(hitInfo.name == "Trigger") 
             {
                 box.isTrigger = false;
             }
         }