It is a lie (or decades out of date) that wages have stagnated. Wages were stagnant from ~1979 to 1997. They have increased a lot since then.

Median usual weekly real earnings: https://fred.stlouisfed.org/series/LES1252881600Q

No for the income tax, when the income tax was introduced in 1913 it was explicitly designed to apply mostly to the rich (off the cuff, I think only a few percent of people paid it for quite some time).

The working poor though earn most their money through wages, which get taxed at the bare minimum 12.4% (payroll + post-payroll SS taxes), and then usually 7-10% sales tax because they are spending all they earn. So the poor are paying 20% tax right off the bat, meanwhile the rich are paying ~20% on capital gains but spending very little of that on sales tax and none on social security. So about the same overall tax rate paid on the earnings of the working poor as the rich.

Social security has always been the main monkey on the back of the working class, to the point the government will literally tax a childless person back into poverty to make sure the quasi-pyramid* scheme is funded. I don't think SS payments are normally recorded in the income tax statistics.

* but technically not