For 2020 this is an "above the line" deduction which means it is used in the calculation to determine AGI.  The calculation of taxable Social Security is based on Modified Adjusted Gross Income.  Since 2020 non-itemized charity is above the line it will impact MAGI which may have an impact on taxable SS.

So if your client is in the zone between 0% and 85% taxable social security benefits then a qualified charitable contribution will make a slight difference in the calculation on the SS worksheet.

Subject to change at any moment, but for now you can look at the draft worksheet on page 30 (pdf page 31) of the draft 1040 instructions:

https://www.irs.gov/pub/irs-dft/i1040gi--dft.pdf#page=31

Line 6 of the worksheet includes an adjustment based on 1040 line 10b (above-the-line charity).

Are we having fun yet?

Rick