Has anyone ever tried to sue the Federal government? Only about a hundred times every frickin' day.
However, when any of them say "Disparate impact," they're saying it's based on the idea that the federal action creates the effect of prohibited discrimination under laws like Title VII of the Civil Rights Act, ADEA, ADA, etc. Treating different types of income differently for policy reasons, or allowing certain types of credits and exemptions for different assets or incomve thresholds, that doesn't even come close to meeting the standard because it may be 'Discrimination' in a dictionary sense, but it's not 'Unlawful' or 'Prohibited' discrimination in a legal sense.