Becuase of parts shortage, today Nikon Japan announced the discontinuation of the Speedlight SB-500 flash and suspension of orders for the Speedlight SB-5000 flash.
Most Nikon Speedlight units have been out of stock in the US for a while (check Adorama and B&H).
As previously reported, I think Nikon is done with developing flash units and has already outsourced that task to third-party manufacturers.
Full text of the announcement (translated):