Don’t it could return.
It was a second brake pedal which allowed the drivers (Hakkinen and Coulthard) to apply braking force to wheels individually (effectively a manual traction control).
I think that it was a photographer from F1 racing spotted that, unusually, the brake discs of the McLarens were glowing through fast corners which would under normal circumstances be flat-out. After waiting most of the season to get his chance, when Hakkinen’s engine expired, the guy took his chance and poked his camera inside the McLaren’s cockpit and discovered an additional pedal.
Following publication of the pictures, Charlie Whiting and the FIA were pretty prompt in pointing out to McLaren that the third pedal was technically illegal.