It wasn't until the late 60's / early 70's that explicit racial discrimination was finally outlawed, and it's not a coincidence that exclusionary zoning took off immediately thereafter as a prima facie race-neutral way to achieve the same outcome.
No, you have the timeline wrong. Exclusionary zoning took off almost immediately after Buchanan. Some of the original designers specifically cited that Supreme Court case as their motivation for passing such laws. Redlining and racial covenants were also used around that time for similar purposes.
As the threat of litigation became a new constant, the San Francisco Planning Department slowly began to craft a new approach to development. The city’s 1971 Urban Design Plan was the first to codify the shift in values from the Modernist freeway-and-tower model toward a greater respect for San Francisco’s unique neighborhoods and their human-scale features. The plan focused on preserving and expanding existing neighborhood character
...
But the largest legislative achievement of this emerging anti-growth coalition would be the Residential Rezoning of 1978, a project to implement stricter controls across all of San Francisco’s neighborhoods. In addition to creating 40-foot building-height limits for most residential areas, the legislation included new setback rules (regulating how far a building could be from the public right-of-way), low-density requirements (limiting the number of housing units in a given building), and overall design guidelines aimed at preserving entire neighborhoods in amber.
That proves that exclusionary zoning was around in 1971, but not that it wasn't used earlier. Massachusetts amended its constitution in 1918, literally one year after Buchanan, to enable cities to impose zoning. And the idea of using it to create racial segregation was specifically discussed at the time [0]:
Of particular concern was the fear that zoning would bring about racial and socioeconomic segregation in Massachusetts, which need not take the form of racial tests, as zoning could just as easily bring about segregation by regulating who could afford certain neighborhoods by income. Pro-zoning advocates... [went ahead anyways]
It wasn't until the late 60's / early 70's that explicit racial discrimination was finally outlawed, and it's not a coincidence that exclusionary zoning took off immediately thereafter as a prima facie race-neutral way to achieve the same outcome.