Must the heirs pay off the accumulated tax debts of the deceased, knowing that these taxes increase with usurious interest?

If there are usurious benefits due to delayed tax payments, these should not be paid. Only the principal tax amount should be paid if the heirs are asked to do so. However, if the taxes themselves are forbidden, then neither they nor their benefits should be paid, unless you are compelled to do so. In that case, the sin falls upon the entity that takes by force of law.