Not sure about the east coast but there's a few big ones out west... KSL 1160 in SLC, KCBS 740 in SF. Also a number around Denver -- KLZ 560, KLTT 670, KLVZ 810, KDCO 1370 off the top of my head. I know KLZ owns the first 3 and possibly LMAs the 4th and if they have any others around CO -- not sure if they own stations in other markets, but they are the only actual group that seems to truly believe in AM Digital on their existing stations. WWFD 820 is all-digital (no analog) and I believe another station is going to do the same... apparently the coverage is much better than analog-only, and the fade and noise issues are gone.
That said, in my corner of the pacific NW, pretty much all stations that ran digital at one time or another have turned it off... KEX, KHHO, KPTR, KUOW-AM have all turned theirs off, and I know there are others I am forgetting.
I remember an older engineer once told me (not sure how correct it is) that AM HD products were only ever made with Gen 1 and Gen 2 exporters and the newer stuff really hasn't been designed for it -- so the reason that you are seeing AM HD just go off the air is that the old exporter goes out and they just don't bother to replace it. Frankly the injection levels were so low at AM digital didn't really make sense because by the time you got to an area with enough noise to disturb the analog signal, the digital signal was long gone. I remember that KEX (50kw) would lose digital signal within about 10 miles, and KUOW-AM (1kw) in about 1-2 miles from the tower -- so that very well could be the truth... I haven't dealt with much AM, and what little I have has not been digital -- so I'm not sure... but makes sense to me. I know there were a number of first adjacent complaints as well.