Mike Piper, author of The Oblivious Investor Blog, explains:
“pre-tax contributions that you make to an employer-sponsored retirement plan such as a 401(k) reduce your income tax, but they do not reduce your Social Security tax. The same goes for traditional IRA contributions, as well as contributions to a SEP or SIMPLE IRA.
And because they have no effect on the amount of your income that’s subject to Social Security taxes, pre-tax contributions to an IRA, 401(k), 403(b), etc. do not reduce the Social Security benefits that you will eventually receive.”
https://obliviousinvestor.com/do-401k-and-ira-contributions-reduce-my-social-security-benefits/
Source: Financial Planning for Women