I looked into the Dutch case. It indeed appears that there's no progressive effective tax, but rather a flat tax. Each group pays about an equal share of the tax burden as their share of the income.

The study published in EBS is a great start, but it is omitting some important elements. Taxation of cars is left out, despite them being a luxury good whose costs are carried by the more affluent class. Transaction tax on real estate, same thing, omitted and mostly a rich-people thing. Real estate taxes, same thing. Nor are capital gains taxes or company's income taxes included, thereby omitting income effects from entrepreneurship and ownership in companies, these capital effects and its taxes of course scale with affluence.

What it does include on the other hand, are various insurances. e.g. insurance to unemployment (including the 'final unemployment': pensions) and sickness. These aren't really taxes per se, but rather individual insurances earmarked per person.

Finally, it hints at combining tax rates with redistribution. For example, healthcare subsidies get deducted from medical insurance premiums in the study. But it's only one of many different forms of redistribution. They don't make similar deductions for rent subsidies (which are quite substantial). I felt that was lacking, either include it all or nothing at all.

Which brings in perspectives... this study essentially adds sales tax to the traditional income tax as a way of determining collective burdens, which is super interesting. But the authors do not change the denominator from income to something else that's related to sales tax. So it's a claim about income+expense tax, as a percentage of income. Rather than say taking all your taxes, including those related to expenses, as a percentage of total expenses. If you did this, you'd see tax systems become progressive again, as rich people have a higher savings rate (i.e., income that's not spent, and thus not hit with sales tax). The authors do mention in the beginning of the article, but don't discuss the implications after. If you wanted, you could see this as a form of deferred taxes.After all, un-spent after-tax income isn't taxed with sales-tax, until it's spent. When its spent, it's taxed, removing the regressive element. And if it's not spent but saved/invested instead, you don't pay sales tax, but you do have to pay tax on capital in the Netherlands. Which is exactly the tax that this study left out!

So, extremely interesting and it's definitely the case that we can't blatantly call the Netherlands progressive without sales tax context. But to make the opposite claim, to say the Netherlands is regressive or flat, probably can't be made as easily either. It really depends on your perspectives.