I'm surprised the bank account is still open.  I don't have any doubt that 2020 decedents (that is, their heirs and beneficiaries) are entitled to the credit, just as they would be on a final 1040 next year had she bought energy-efficient windows in January.  For 2019 decedents, I think IRS had to follow the precedent from 2008, when they sent rebates to those who died in 2007.  For 2018, I think IRS asked Treasury, "Do you want it right or do you want it fast?", and they got their answer.