Is it permissible to donate money on behalf of a person who died a long time ago, whose heirs and grandchildren have also passed away except for a large number of them, without informing the remaining heirs, knowing that each individual's share would be very small?

Ownership of wealth transfers to the heirs, and it must be given to them to clear one's conscience. It is not permissible to donate it on behalf of the deceased unless they permit and consent to it, because the Prophet, peace and blessings be upon him, said: "The wealth of a Muslim person is not lawful except with his willing consent."