I just went to IRS and checked -- it is true about 2021. Also, however, for 2022 tax year, the 400% limit does not apply. However this special rule only works for 2021 and 2022. So if your AGI in 2023 is going to be over 400% of FPL, then you're not allowed any Premium Tax Credit and must repay the entire amount you used. So plan accordingly -- if you know you will make more than 400% FPL, don't claim the PTC for 2023.
