The CARES Act allows you to defer paying part of the Social Security tax for your household employee, if certain requirements are met. The deferral appears as a refundable credit on Schedule 3 line 12e. It was subtracted from the tax you had to pay with your tax return. Half the deferred tax is due December 31, 2021, and the remainder is due December 31, 2022. The deferral is only allowed for 2020.
The IRS letter is about the self-employment tax on your tax return, not Social Security tax that is withheld from your wife's pay by her employer. If any of her Social Security tax was deferred, it was done by her employer, not by you. If the employer deferred any of the Social Security tax, the employer will collect the deferred tax in 2021.
See the following IRS notice for instructions to pay the deferred tax for your household employee.
How self-employed individuals and household employers repay deferred Social Security tax
