Open Data Portal

https://data.cityofnewyork.us/Housing-Development/Scenic-Landmarks/gi7d-8gt5

This dataset is regularly updated and includes descriptive info. Includes both designated and "calendared" (under consideration) scenic landmarks.

ArcGIS REST Services

As of winter 2020 the Landmarks Preservation Commission publishes ArcGIS REST data services supporting the Discover New York City Landmarks application, including Scenic Landmarks at this (possibly password-protected) endpoint:

https://services5.arcgis.com/Oos4pNA2538iVFA1/ArcGIS/rest/services