Bat and ball game - is there a way one can store the ball's last collision?

 Collision lastCollision;
 
 void CheckLastCollision()
 {
     if(lastCollision)
     {
         if(lastCollision.gameObject.name == "Bat")
         {
             // Collided with the bat last time
         }
     }
 }
 
 void OnCollisionEnter(Collision coll)
 {
     lastCollision = coll;
 }

This creates a 'lastCollision' variable, which gets updated at every collision. You can check the last collision with the CheckLastCollision() function. If you want to check it on collision, be sure to write it at the beginning of the OnCollisionEnter void, so it will check the last collision first, then assign the new collision to it.

By the way, you can use

 Collision[] lastCollisions;

to create an array of collisions, if you want to check a collision history. Be sure to initialize the array in the Start() method with

 lastCollisions = new Collision[length];

and replace 'length' with a number. The update of this array is the following:

 for(int i = 0; i < lastCollisions.Length - 1; i++)
 {
     lastCollisions[i] == lastCollisions[i + 1];
 }
 lastCollisions[lastCollisions.Length - 1] = coll;

The function will first replace every collision in the array (starting from index 0) with the next collision in the array. Then the last one is replaced with the fresh collision ('coll').

However, the array has a fixed size. If you want a dynamic-sized container, you could use lists.

 using System.Collections.Generic;
 
 List<Collision> lastCollisions = new List<Collision>();
 
 lastCollisions.Add(coll);
 lastCollisions.RemoveAt(index);

The first line will create a list with the type of Collision, so it can hold only collisions. The second line can be used to add a collision to the list (the variable is named with 'coll', and it will be added to the end of the list). The third line can be used to remove a collision in the list with an index number.