I have had the same thought TBH.
The East Coast, as primitive to the NCR as it is, still has trade routes going all the way from North Carolina, all the way north to Boston, as far west as Pittsburgh, and as far north-west as Ronto.
NYC lies within that area, and no one mentions it at all. Even Philadelphia got a mention of bring a ruin, and was called "that dump". If NYC survived in any way, one would think it would have gotten SOME mention by people in D.C., the Pitt, or even Zimmer, since he would have had to pass it on his way to D.C., but nope, it goes totally unmentioned.
The only mention of NYC in-game, that I know of, is in the Fallout 4 E3 vids, where the TV news guy says they got confirmed reports of nukes hitting NYC, this being moments before The Sole Survivor and his wife run off to reach Vault 111.