Well, I suppose the first thing I would try is to soak it in penetrating oil for several days. Put some heat to it from time to time, maybe lightly tap the ends of the shaft, all trying to get any movement at all.
Once there is even a little movement, then you know that the oil will be able to continue to get deeper.
Keep working it and I expect it should eventually loosen up.
Another option might be to grind away the spot welds that hold the flap to the shaft, and then press the shaft out, but I'd still make every effort to get the shaft loose first. Once it is all apart and cleaned, when re-assembled, just re-weld the plat to the shaft.