Slide when touching walls without losing velocity

Quite tricky thing.

First of all, you'll need to use Rigidbody's OnCollisionStay() event to check if collided with a wall (or something else that has its surface positioned at ~90 degrees to the ground).

To do that, you'll use the GetContacts property of the Collision, iterate through them and find out those contact points where the normal (a vector that is perpendicular to the surface) is at ~90 degrees to the ground normal. If Collision has such points, then it means you are touching a wall.

 ContactPoint[] points = new ContactPoint[20];
 Vector3 wallNormal; //this will store the vector that points out from the wall
 bool isTouchingWall;
 
 OnCollisionStay(Collision col)
 {
     int pointsAmount = col.GetContacts(points);
     isTouchingWall = false; //don't know if we're near a wall yet
     for (int i = 0; i < pointsAmount; i++)
     {
         //if angle between the touched surface and the plane ground is more than 80 degrees, than it means this surface is kind of a wall
         if (Vector3.Angle(points[i].normal, Vector3.up) > 80f)
         {
             wallNormal = points[i].normal;
             wallNormal.y = 0f; // eliminate all possible slopes of the wall touched so the vector will be ideally horizontal.
             isTouchingWall = true;
             break; //quit the iteration as we already got some wall
         }
     }
 }

So, now if the rigidbody has detected some wall, isTouchingWall will be true. But what if we jump? In this case, OnCollisionStay() won't be triggered, and isTouschingWall might stay true. Set it to false in:

 OnCollisionExit() {
     isTouchingWall = false;
 }

Now, we know when the rigidbody is near a wall, and we know the wall normal vector.

With this info you'll need to change your move direction that is applied to the rigidbody in your movement methods. The resulting direction should point along the wall surface. To get this new direction you'll need to use Vector3.Reflect to get the opposite version of the regular move direction (kind of reflected off the wall surface). And than multiply the regular move direction with the reflected direction to get the centered direction that is pointing in the middle of them, that will be the direction along the wall.

 void Move()
 {
     //you've calculated your move direction as usual
     moveDirection = ... // your code
     // and than you should change it so it will point not in the wall, but along the wall
     if (isTouchingWall)
     {
         //get the direction that will be your move direction reflected off the wall
         Vector3 reflectedDir = Vector3.Reflect(moveDirection, wallNormal);
         //and get the middle center direction between the two
         moveDirection *= reflectedDir;
         rb.AddForce(...); //your code to move the rigidbody
     }
 }