I believe in 1983 by Saint Pope John Paul to underline the gravity of abortion. 

It's not a "condition on forgiveness" but on removing the canonical penalty of excommunication. In the eyes of the Church, abortion involves both a grievous sin and a canonical crime. Prior to 1983, a priest could absolve the sin and remove the excommunication. Plus, it only applies if the person was aware of the penalty of excommunication prior to having or cooperating in an abortion.