Slightly more specific text from https://www.nyc.gov/html/dot/downloads/pdf/trafrule.pdf --

(i) Marked crosswalk. The term “marked crosswalk” means that part of a roadway defined
by two parallel lines or highlighted by a pattern of lines (perpendicular, parallel or diagonal
used either separately or in combination) that is intended to guide pedestrians into proper
crossing paths.
(ii) Unmarked crosswalk. The term “unmarked crosswalk” means that part of a roadway,
other than a marked crosswalk, that is included within the extensions of the sidewalk lines
between opposite sides of the roadway at an intersection, provided that (A) the roadway
crosses through the intersection rather than ending at the intersection, and/or (B) all traffic
on the opposing roadway is controlled by a traffic control device.

The way I'm reading this, an "unmarked crossing" is valid (curb ramps or no) if all of the following conditions are met:
a) There are no crosswalk markings.
b) It's at a "+" intersection, not at "T" intersection.
c) The sidewalk continues on the other side of the intersection.
d) The intersection features a stop light or stop sign controlling traffic on the road being crossed.

Offhand I'd say this is a much saner definition than the one in https://www.nyc.gov/html/dot/downloads/pdf/hwyrules.pdf ... and on closer reading the context there is newsrack regulations, not pedestrian traffic.