As any bishop emeritus, Benedict XVI retains all his episcopal faculties related to the sacraments as a validly ordained minister of the Church. Nevertheless, with his resignation, he lost all the powers related to his office, so he cannot exercise the supreme authority of the Papacy. The
munus regendi can only be exercised with the mandate of the Supreme Pontiff and being in communion with the Holy See. Therefore, he CAN confect any sacrament as he can act
in persona Christi. He just can’t write Encyclicals, designate Bishops, create Cardinals, etc.
There’s still one issue left unclear. To celebrate the sacraments validly, any minister needs jurisdiction, which is to be obtained from the Ordinary. As Bishop of Rome, the jurisdiction of the Pope is universal. The question is: as a Pope emeritus (regardless of the actual existence of that title), does he retain the universal jurisdiction? I think not, since it comes attached to the office of Pope. Hence, as the Bishop Emeritus of Rome, he would only have jurisdiction over his former diocese as is the case of any retired bishop. Ergo, he can celebrate the mass, hear confessions, etc, within the limits of the diocese of Rome. Although, as he himself has said, he now is hidden from the world. Because of that, I guess he won’t celebrate any ordinations, baptisms, not even hear confessions or anoint the sick or have any kind of public ministry.
But this is only hypothetical. Let’s leave the canonical issues to the experts
Greetings and God bless!