Is the Bank responsible for alleged unauthorized ACH transactions on a closed account ACH dispute where the account has been closed well beyond the retention requirements of the Bank (for example 15+ years) and the account records aren't available any longer?

I sincerely hope you're asking a hypothetical question. So I'll say, hypothetically, yes. There is no statute of limitations on Reg E claims that an EFT was unauthorized. But I would not want to be a judge trying to determine the right or wrong of your decision.