Wikipedia, interestingly, notes that the 13th century hymn, the famous Dies Irae warning the people of God to reform their lives before it is too late, has been dropped from the New Liturgy:
The "Dies Irae" was used in the Roman liturgy as the sequence for the Requiem Mass for centuries, as evidenced by the important place it holds in musical settings such as those by Mozart and Verdi. It appears in the Roman Missal of 1962, the last edition before the implementation of the revisions from the Second Vatican Council. As such, it is still heard in churches where the Tridentine Latin liturgy is celebrated. It also formed part of the traditional liturgy of All Souls' Day.
In the reforms to the Catholic liturgy ordered by the Second Vatican Council, it was retained only in part by the "Consilium for the Implementation of the Constitution on the Liturgy"—the Vatican body charged with drafting and implementing the reforms (1969–70). It remains as a hymn ad libitum in the Liturgy of the Hours during the last week before Advent, divided into three parts for the Office of Readings, Lauds and Vespers.
Nevertheless, the Consilium felt that the funeral rite was in need of reform and eliminated the sequence as such from the Masses for the Dead. A leading figure in the post-conciliar liturgical reforms, Archbishop Annibale Bugnini, explains the mind of the cardinals and bishops who were members of the Consilium:
They got rid of texts that smacked of a negative spirituality inherited from the Middle Ages. Thus they removed such familiar and even beloved texts as the "Libera Me, Domine", the "Dies Irae", and others that overemphasized judgment, fear, and despair. These they replaced with texts urging Christian hope and arguably giving more effective expression to faith in the resurrection.
Ab. Bug____ could not have been a Catholic. No Catholic would ever allow that anything in the Catholic liturgy or Faith could have "overemphasized judgment, fear, and despair".
Beware lest any man cheat you by philosophy and vain deceit: according to the tradition of men according to the elements of the world and not according to Christ.