"On this day the sacrifice of the sacred Body and Blood of the Lord was initiated as a celebration by the Lord himself. On this day the holy chrism is consecrated throughout the world; on this day too pardon is granted to penitents, those at enmity are reconciled, those who are aggrieved are pacified" (Letter of Pope Sylvester, 4th century).
The Rite of Reconciling Penitents remained in the Roman Pontifical until the 20th century, but the reality had long since lapsed centuries earlier with the decline of public penance and the rise of private confession. In The Liturgical Year, volume 6, Gueranger gave a detailed description of the ancient ritual and of the Papal Blessing that used to be given from the inner loggia of Saint Peter's on this day.
But if the Reconciliation of Penitents long ago passed into. history the other two ancient aspects of Holy Thursday have successfully survived into the present. While a separate Chrism Mass had long since disappeared, over the centuries the blessing of the Oil of the Sick, the Oil of Catechumens, and the Sacred Chrism had become an integral part of the Pontifical Mass of the Lord's Supper celebrated at the Cathedral on Holy Thursday morning. Pope Pius XII's Holy Week reform in 1955 recreated the ancient Chrism Mass and assigned it to Holy Thursday morning, while moving the traditional Mass of the Lord's Supper from morning to early evening both in the cathedral and in parish churches. The Chrism Mass was radically revised as part of Paul VI's post-conciliar liturgical reform and has since acquired an unprecedented degree of popularity, particularly among clergy.
For the most part, "private" Masses have been prohibited on this day, with all priests encouraged to participate (and, since Vatican II, to concelebrate) at the one solemn community Mass., which since 1955 has been celebrated in the evening. Generally this has been my favorite of the Holy Week services, and I will especially miss celebrating it this year.
Other than the unfortunate transfer of the reading from Exodus 12 from Good Friday to Thursday, the other significant alteration in the post-conciliar Mass of the Lord's Supper was the elimination of verses 20-22 and 27-32 from the traditional reading from 1 Corinthians. The omitted verses frame and contextualize the institution narrative, which is all that now remains of the traditional reading. In recent years, as divisions within both the Church and society have increased (with the latter amplifying and further fueling the former), I have regularly referred to those missing verse in my Holy Thursday homilies to better relate Saint Paul's challenge to his contemporaries to our present situation. So, for example, as I said two Holy Thursdays ago in 2019:
(Photo: Altar of Repose, Immaculate Conception Church, Knoxville, TN, Holy Thursday 2019.)