The title of this piece is “Effective Income Tax Rates Have Fallen for The Top One Percent Since World War II” which seems to directly contradict your statement they have been stable?
Right—they fell sharply from 1945 to 1955 (it looks the data points are sampled every 10 years) but have been pretty stable in the post-war era, as I said. Note that OP stated the top tax rate was lowered starting in the 1970s.
How is dramatically lowering since 1970s compatible with stable after 1955? The fact is tax on the rich started going down in the 1970s and since the 90s tax havens and other shenanigans like renting out your own mega yacht to off shore shell companies even though you were using it. And a million other ways these people don’t pay.
For the richest tax is optional to the point where that get loans against their shares rather than pay capital gains tax. It’s an absolute disgrace and it’s got to a point where governments need to taxing them properly.
The effective tax rate on the top 1% has been quite stable in the post war era.