The Trust fund (FICA tax) was and is separate from the general revenue (income tax) - Ronald Reagan was the first president to tap into the trust funds to meet the federal debt, 1981 amendments.
The Republican party 1983 amendments - Ronald Reagan signed the bill for taxing Social Security benefits. Had to be done to replace money used to pay the national debt.
In the 1993 amendments, the Omnibus tax bill, Public Law No: 103-66, taxed SSA benefits for those with other income over $20,000 a year. It had to be done as the funds were going broke because of being used to fund the national debt.
Public Law 92-603, Supplemental Security Income (SSI) the 1972 amendments signed by Richard M. Nixon.
American citizens can get SSI without ever having paid a dime into Social Security, FICA tax, SSI is welfare administered by the federal government through the SSA. Elibility is based on age, over 65, blindness, and disability, and financial need.
A Brief History of SSA and Amendments