It is legit, although it might not apply to you. Do you have self employment income? Check if you have Schedule 3 line 12e. The IRS has also been mailing letters to remind people to pay their half by Dec 31.
The deferred tax appears as a refundable credit on Schedule 3 line 12e, "Deferral for certain Schedule H or SE filers." It was subtracted from the tax you had to pay with your tax return. You have to pay half the deferred tax by December 31, 2021 (the end of this month), and the remainder by December 31, 2022.
