Is it permissible for the Imam to recite Istighfar (seeking forgiveness) aloud and collectively after finishing the prayer?

Reciting supplications aloud in congregation after the five prescribed prayers, or after the supererogatory and regular sunnah prayers, on a continuous basis, is a reprehensible innovation (bid'ah munkarah). This is because it has not been authentically reported from the Prophet (peace be upon him) nor from his companions (may Allah be pleased with them).