I think the rotted board is not supporting the bricks, if those bricks are full size bricks. There's probably some metal support (I-beam?) behind the rotted board, supporting the brick wall (and maybe partially the arch).

To me, the (upright) brick arch is simply decoration, probably not full bricks. Its arc is not supporting the brick wall and that should be obvious by the fact that the bricks adjacent to the keystone have been cut, inappropriately, defeating the purpose of a true keystone function. If the rotted board was for support, then likely the bricks and mortar, above, would have cracks, somewhere. The rotted board, I suspect, is simply a space filler.