At every station I've ever been to, the boarding is level, and the gap is small.
Many (though not all yet) stations have elevators large enough for a disabled person in a wheelchair.
All buses (operated by the same MTA as the usbway) have a foldable ramp for wheelchairs, used quite routinely.
> 14th Street–Union Square northbound 4 trains have a 5.5-inch vertical gap, 59th Street–Columbus Circle northbound B trains have a gaping seven-inch horizontal gap, and Times Square–42nd Street eastbound 7 trains have both: a 5.5-inch vertical gap and a five-inch horizontal gap
https://www.curbed.com/2022/10/mta-subway-platform-gaps-laws...
Platforms can sink and otherwise decay over time. Many of the stations are quite old.
Low platforms, by contrast, are nowhere near the height of the train floor.