Quick google search online brought up a paper arguing this point. The key point though might be this:

"Update: 10-29-21 - This topic has been to the U.S. Supreme Court as a Petition three times now, and three times the court has denied hearing their own rulings being discarded and illegal activities by the IRS allowed to continue. 19 court cases to date and all denied due process."

From what I can tell, the IRS made up a whole new category called "capital gains tax" in order to keep the ruse alive. Capital gains ARE income. Wages are not. But by creating a separate category for capital gains income, they can fool us into thinking wages count as "regular income." Something that, legally, does not even exist. All income is a capital gain. Wages are not income. You can choose to save your wages and invest them, and what you earn minus the principal after the sale of your investment can legally be taxed, but not the principal itself, which came from your wages.

But as we know, the IRS doesn't give two shits about what's technically legal. They're out to get your money, period. They know nobody wants to be the one guy who asserts his rights with no confidence that he'll be backed up by the courts, so they keep it going. Now THAT would be a case I would hope SCOTUS takes up and strikes down, because then we could probably approach the critical mass necessary to throw off the scam collectively all at once. It wouldn't even matter if they tossed out the 16th amendment because they've already stated that government already had the authority to tax income. Until we all learn what income really is, we'll continue to give the IRS a part of our salary that they don't have any legal right to.