Open Data Portal

https://data.cityofnewyork.us/Housing-Development/Individual-Landmarks/ch5p-r223

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

Interior landmarks are a subset of individual landmarks. They are interior in the sense that they are inside, not outside.

In the Open Data dataset points with lm_type = ‘Interior Landmarks’ are 'interior landmarks.'

ArcGIS REST Services

As of winter 2020 the Landmarks Preservation Commission publishes (possibly with restrictions) ArcGIS REST data services supporting the Discover New York City Landmarks application, including Scenic Landmarks here:

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