Thanks. I suspect they will ask for 2019 tax returns to confirm the income I have entered in the application to determine that I'm actually eligible for the subsidy. Is there a form that I also submit to explain the big difference between the taxable income in 2019 and the significantly smaller amount of income I expect in 2021?
Surprisingly they don't, at least not in my state. They just ask you to say how much you earn from work, how much from investments, and how much from retirement/pensions.
As far as explaining big differences, they pretty much always accept a simple letter of explanation. "Dear ACA people, I'm going to make less in 2021 because I retired on 12/25/2020. Attached please find my resignation letter to my employer and a worksheet showing my taxable dividends and Roth conversion taxable income. Sincerely, Omy"
FWIW, I tried to go into great detail with them about my financial situation the first year because I was ER'd and wanted to explain a lot to make sure we were all on the same page. After a few years, it really seems to me that they just want to make sure that any subsidy paid out is in the ballpark so that taxpayers won't get mad at them at tax time if they have to pay a lot back because their subsidy was too big. So nowadays I just tell them, "Hey, my retirement income is going to be $X, mark it down, final answer." And they do. In truth my income is from a variety of sources including wage and investment income, but since the math works the same nobody cares.
So if you're sure of your numbers and have some sort of evidence, generally they'll accept whatever you say. (Note that this can vary from state to state, but from what I've read of others' experiences it generally matches mine even if they're in other states or dealing with the federal exchange.)