Contributions to most qualified retirement plans are not tax-exempt, they are tax-deferred. When you make a qualified contribution to one of those plans, you don't have to include the amount of the contribution in your taxable income for that year. That has been in place under the U.S. tax code since the 1970s, and it remains so to this day.
What changed during the Clinton administration was the introduction of a new kind of retirement plan where the contributions were not tax-deferred, but the withdrawals in retirement were tax-exempt. That's what the Roth IRA is.
*sigh* Before Clinton your SS payments were not taxed. When you retired those payments were taxed at a lower rate. Clinton had your contributions taxed >first< at the higher rate while you were working .....