New Advent goes to great lengths to inform us of the RCC catechism which states that no one can be excommunicated after they die.
Sort of.
See post 70.
People can be judged guilty of heresy, etc., after they're dead.
But it would make no sense to excommunicate someone after they're dead. An excommunication is formally depriving someone of their right to receive the sacraments. Dead people don't receive sacraments; sacraments are only for the living.