Is it obligatory to pay Zakat on shares for past years (10 years), given that they were sold 3 months ago and I do not know their number or market value during that period?

The method of paying Zakat on shares has been previously explained. If the shares are fixed assets not intended for trade, there is no Zakat on their principal value. Rather, Zakat is due on their revenue if it reaches the nisab and a year has passed over it. If the shares are commercial goods, then Zakat is due on their entire value. In this case, one should ascertain the value of the shares at the beginning of each year since owning the nisab and pay Zakat for all past years. Assistance can be sought from the company to determine the number of shares and their value.