How can I get the lowest position of objects that are visible by the camera?

It would go something like this. Loop through all platforms, test their transform position plus their bounds to find the maximum distance in your left/right direction. Get the points on the left/right side of screen where the camera's view frustum intersects the objects (i.e. use some trigonometry to cast a ray from the camera's position along its view frustum wall (which is conveniently an angle of half of the FOV) to find the world position in the same plane as your platforms. If the platform is currently visible in your view, determine the maximum DOWN distance from the platform (using its bounds). Now add some fudge factor just below that for the death area. In the next iteration, check if the current platform is lower than the death area and adjust the death area as needed. Optimally, you would just keep a single Y value through the loop that is only allowed to go down, then set the death area's position to that Y value (assuming you are using Y for up).