Before that, the same Church prohibited private confession.
That’s not really true. Private confession has always been the norm, since the very early Church. Actually, after privately confessing your sins, the Priest would decide if it was necessary to also confess them in front of the congregation. This was often done in cases where the sin was considered to be particularly scandalous for the people, or serious enough for it to be made public (murder, etc.). Your penance was also performed in public.
From New Advent
"The Didache written at the close of the first century or early in the second, in 4.14 and again in 14.1,
commands an
individual confession in the congregation:
In the congregation thou shalt confess thy transgressions"; or again: “On the Lord’s Day come together and break bread . . .
having confessed your transgressions that your sacrifice may be pure.” Clement I (d. 99) in his Epistle to the Corinthians not only exhorts to repentance, but begs the seditious to “
submit themselves to the presbyters and receive correction so as to repent” (chapter 57), and Ignatius of Antioch at the close of the first century speaks of the mercy of God to sinners, provided they return" with one consent to the unity of Christ and the communion of the bishop". The clause “communion of the bishop” evidently means the bishop with his council of presbyters as assessors. He also says (Letter to the Philadelphians) “that the bishop presides over penance”.
…
"The penitential process included a series of acts,
the first of which was confession. Regarding this,
Origen, after speaking of baptism, tells us:
“There is a yet more severe and arduous pardon of sins by penance, when the sinner washes his couch with tears, and when he blushes not to disclose his sin to the priest of the Lord and seeks the remedy” (Homil. “In Levit.”, ii, 4, in P.G., XII, 418). Again he says: “They who have sinned, if they hide and retain their sin within their breast, are grievously tormented; but if the sinner becomes his own accuser, while he does this, he discharges the cause of all his malady. Only let him carefully consider to whom he should confess his sin; what is the character of the physician; if he be one who will be weak with the weak, who will weep with the sorrowful, and who understands the discipline of condolence and fellow-feeling. So that when his skill shall be known and his pity felt, you may follow what he shall advise. Should he think your disease to be such that it should be declared in the assembly of the faithful—whereby others may be edified, and yourself easily reformed—this must be done with much deliberation and the skillful advice of the physician” (Homil. “In Ps. xxxvii”, n. 6, in P.G., XII, 1386). Origen here states quite plainly the relation between confession and public penance. The sinner must first make known his sins to the priest, who will decide whether any further manifestation is called for."
