Here is information from the Centers for Medicare & Medicaid Services about the changes in the ACA from the American Rescue Plan Act of 2021.
This applies to 2021 and 2022, and did not mention any retroactive changes to the ACA.
What you're referencing is new rules regarding eligibility for ACA subsidies for this year and next. What I was referencing specifically pertains to guidance regarding waiving subsidy payback for 2020 subsidies when income for 2020 is higher than estimated during ACA enrollment for 2020. This waiver has been highlighted in multiple sources, so I'm pretty sure this is part of the Stimulus package. I was told earlier today by one of your phone reps that I'll need to amend my return in order to get a refund from the IRS and should try this after 3/26/21.