Is it obligatory to pay a debt owed by the deceased, which one of the heirs knew about, but the creditor did not claim until 15 years after the debtor's death?

The father's debt must be paid from his estate, if proven by evidence or admission. The heirs are obligated to fulfill it from what they have divided, without nullifying the division. If the father did not leave enough to cover the debt, you are not required to pay it, but it is commendable to do so as an act of filial piety.