How is income tax regressive?
No. A flat tax is proportional [1].
[1] https://www.investopedia.com/ask/answers/042415/what-are-dif...
> While a flat tax imposes the same tax percentage on all individuals regardless of income, many see it as a regressive tax.
> A regressive tax imposes a larger tax burden on those with lower incomes than those with higher incomes because a larger portion of a lower-income individual's total funds goes to the tax. While higher-income taxpayers pay the same percentage, their greater incomes make the financial burden easier to bear.