The SE tax deferral is a credit on the 2020 tax return, so presumably the repayment would have to be reported as an additional tax on Schedule 2 of the 2021 and 2022 tax returns. This will likely require TurboTax to carry forward the amount of the credit yet to be repaid (or to at least ask for that information to be supplied by the user). Still, that doesn't explain how the 12/31 repayment deadlines would be enforced.
