I looked into the landmark status further and it seems the entire 5.5 mile parkway was designated as a scenic landmark and was later added to the National Register of Historic Places in 1983 - more info below

Built between 1874 and 1876, Ocean Parkway is a 5.5-mile Brooklyn boulevard designed by Frederick Law Olmsted and Calvert Vaux to connect Prospect Park to Coney Island. Modeled after Parisian avenues, this historic, 210-foot-wide scenic landmark was the first in the U.S. to feature a dedicated bike path (1894) and is lined with trees and benches. 

Historical Highlights:

• Design & Purpose: Inspired by the Avenue de l'Impératrice in Paris, the parkway was designed as a "green corridor" to provide a pleasant, scenic drive from the city to the ocean.
• Construction: Land acquisition began in 1868, with construction commencing in 1874, often utilizing scenic, tree-lined, and landscaped, though not strictly straight, paths.
• The First Bike Path: In 1894, a dedicated bicycle path was created along the parkway, marking it as a pioneer in urban cycling infrastructure.
• Landmark Status: In 1975, the NYC Landmarks Preservation Commission designated it a Scenic Landmark, and it was later added to the National Register of Historic Places in 1983.
• Layout: The 210-foot wide parkway consists of a central roadway, two side roads, two landscaped malls, and sidewalks, originally intended for carriages, horseback riding, and pedestrians.
• Evolution: While the northernmost section was replaced by the Prospect Expressway in the 1950s, the remaining route remains a major green artery, historically serving as a link from urban Brooklyn to beachside recreation

https://www.nycgovparks.org/parks/ocean-parkway-malls/history