The first Social Security benefits were paid in 1940. From that time until 1984, benefits were exempt from federal income tax, as authorized by Treasury Department rulings issued in 1938 and 1941 (SSA n.d.).
Because other forms of retirement income (such as private- and public-sector pensions) were subject to income tax, policymakers eventually reconsidered the tax exemption for Social Security benefits. Both the 1979 Advisory Council on Social Security (1979) and the National Commission on Social Security Reform (1983) recommended that some Social Security benefits be included in taxable income. ....
.... When the 1983 amendments went into effect, about 8 percent of beneficiary families were required to pay income tax on part of their Social Security benefits (House Ways and Means Committee 2004). That percentage has increased over time because the 1983 amendments set the thresholds for taxation of benefits in nominal dollars, rather than indexing them to price or wage changes in the national economy. By 1993, an estimated 20 percent of beneficiary families paid income tax on part of their benefits (Pattison and Harrington 1993). Subsequent estimates by the Congressional Budget Office (CBO) put the percentage of beneficiaries paying income tax on their benefits at 25 percent in 1997, 32 percent in 2000, and 39 percent in 2003. More recently, CBO estimated that 49 percent of Social Security beneficiaries paid income tax on their benefits in 2014
https://www.ssa.gov/policy/docs/issuepapers/ip2015-02.html