https://dissidentvoice.org/2015/06/greenspan-finally-tells-the-awful-truth-about-social-security/
"Over a 30-year period, the $2.7 trillion in surplus Social Security revenue, generated by the 1983 payroll tax hike, was taken from Social Security and spent as general revenue. As the money was spent, it was replaced, dollar for dollar, with non-marketable government IOUs called special issues of the Treasury. These are government IOUs which are not marketable. They couldn’t be sold to anyone, even for a penny on the dollar."
https://www.taxpolicycenter.org/briefing-book/are-social-security-trust-funds-real
"So, are the trust funds real? Yes. They have legal consequences for the Treasury and are backed by the full faith and credit of the federal government, just like other Treasury bonds. When the Social Security Administration redeems the bonds, the government has a legal obligation to pay the money back with interest, with no additional appropriation by Congress required.
The trust funds are not a free lunch for taxpayers. Money from the general fund used to repay debts to the trust funds cannot be used for other purposes, like building roads or providing for national defense. And as an additional outlay for the government,
those general fund payments increase the Treasury’s need to borrow from the public, increasing federal deficits and adding burdens on future taxpayers."