Quickfinder: “If the child turns 18 before the adoption is finalized, only expenses incurred before the child turned 18 will be eligible for the credit. 

Exactly.  The age of the child when adopted doesn't matter, because you're not claiming the child.  What matters is the age of the child when the expenses were incurred.  The child could have turned 18 in 2020 and if the adoption was not final until 2021, the expenses incurred before his birthday in 2020 would still qualify on the 2021 return.   

You criticized the software in your initial post for possibly allowing a credit for expenses that don't qualify.  However, asking for the child's date of birth would not make the program any more accurate.  The preparer must still know the law.