Will the Turbo Tax federal refund account for SS tax not being withheld since September due to the tax deferral? Or will I have to plan on an additional amount for that?

Social Security taxes paid are not part of your federal tax return, unless you paid excess SS taxes from two or more employers.

If your employer offered the social security deferral, you still owe those taxes.  (Unless Congress passes a new stimulus law and forgives them.)

Under current IRS guidance, your employer is required to withhold those deferred taxes from your pay in equal installments between January 1, 2021 and April 30, 2021.  This is not a situation that will appear on your federal income tax return.

If your employer does withhold those deferred taxes in 2021, you would expect to receive a corrected W-2C some time in 2021.  If the only thing that changes is the box 4 social security number, you will not file an amended tax return. 

https://www.irs.gov/pub/irs-drop/n-20-65.pdf

https://www.journalofaccountancy.com/news/2020/oct/employee-payroll-tax-deferral-irs-guidance.html