> SCOTUS has found non-protected categories can still be protected because they are "proxies" for protected categories. One of the classic examples of this are zip codes[0], which was found to be a proxy for race, because it has a "disparate impact" on people of particular races.

This was probably wrong, both in terms of interpreting the existing law and as a statement of what the law should be. Sometimes bad facts correlate with race; that should not be a reason to deny using the measure for e.g. hiring or lending.