Ive seen this same question asked multiple times to day in different groups, I havent seen an answer though. 

It says due by 12/31/21, Im guessing youd send in a 1040V voucher maybe?  I have no clue, I havent researched it at all.  I dont have anyone thats wanted to defer.

At the moment that question is at the bottom of the IRS in-basket, but they will probably have some sort of answer by Thanksgiving, provided there are no government shutdowns or domestic terrorist attacks before then. They are promising reminders to employers who pay 941 taxes, but those can be done with tax deposits.  Here is what IRS says about those:

20. Will the IRS issue reminder notices to taxpayers reflecting the total amount of deferred taxes and the payment due dates? (added July 30, 2020)

The IRS intends to issue a reminder notice to employers before each applicable due date. Because each return period is treated separately for purposes of determining the amount of tax due for the period, Form 941 filers that deferred in all four quarters of 2020 may receive four reminder notices stating the deferred amounts that are due on the applicable dates in 2021 and 2022, even though the amounts for all four quarters will have the same due dates of December 31, 2021 and December 31, 2022.

https://www.irs.gov/newsroom/deferral-of-employment-tax-deposits-and-payments-through-december-31-20... 

They have decided...not how I expected. From IRS.gov:

"How individuals can repay the deferred taxes

Individuals can pay the deferred amount any time on or before the due date. They:

• Can make payments through the Electronic Federal Tax Payment System or by credit or debit card, money order or with a check.
• Should be separate payments from other tax payments to ensure they are applied to the deferred tax balance on the tax year 2020 Form 1040 since IRS systems won't recognize the payment for deferred tax if it is with other tax payments or paid with the current Form 1040.
• Should designate the payment as "deferred Social Security tax."

Individuals making deferred Social Security tax payments in EFTPS should select 1040 US Individual Income Tax Returns and deferred Social Security tax for the type of payment. They must apply the payment to the 2020 tax year where they deferred the payment. Taxpayers can visit EFTPS.gov for details."

https://www.irs.gov/newsroom/how-self-employed-individuals-and-household-employers-repay-deferred-so...

@BobKamman That assumes the IRS has processed the 941s.

"As of July 2, 2021, we had about 5,000 Forms 941 received prior to 2021 in the processing pipeline. Including current year returns, as of July 2, 2021, we had 1.6 million unprocessed 941s in the pipeline. "