What is the ruling on the zakat due on the estate of a deceased father, knowing that the heirs did not pay it for 20 years out of ignorance, despite having given a lot of money from the estate in charity?

Zakat is not waived due to ignorance or the passage of time. Every heir must pay zakat on their share if it reaches the nisab and a hawl has passed on it. Zakat for each year is estimated based on when the wealth reached the nisab. Real estate is not subject to zakat unless it is for sale, or on its rent if a hawl has passed on it. If shares are for sale, zakat is on their value and profits. If they are for benefiting from their yield, zakat is only on the profits. Charity does not waive obligatory zakat.