It's the coach bulkheads on full size airplanes (MD88s, 737s, 767s) that I've never seen available.
I know that at the end of this past May I was assigned bulkheads in coach on the 767-300 and on the 767-200. I was seated in the second coach cabin. Now the bulkheads in first coach cabin on these planes have never been available to me as they are Medallion seats and are usually picked over by the time I buy my ticket anyways. The bulkheads in coach usually go quickly, so the sooner you can buy your ticket the better.
I just went to my next itinerary and this is what it says below the seat selection tool:
Delta accommodates the seating requirements of customers with certain types of disabilities. This could result in the occasional need to change another individual's pre-assigned seat, with bulkhead seats being particularly subject to reassignment. We appreciate your cooperation in these special circumstances.