Forgiven Mortal Sins and Venial Sins still require reparation for our actions in Purgatory. Confessing venial sins is recommended.
(Ref: indulgence)
CCC - " 1472 To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence.
Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the “eternal punishment” of sin.
On the other hand every sin, even venial, entails an unhealthy attachment to creatures,
which must be purified either here on earth, or after death in the state called Purgatory.
This purification frees one from what is called the “temporal punishment” of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin.
A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain."
For a Plenary indulgence one must go to Confession even though not in a State of Mortal Sin.
One can only obtain a Plenary indulgence for himself, or someone who is deceased and in Purgatory.
This is an example.
divinemercysundayusa.com/bulletininsert11.pdf
God has great Mercy for those seeking his forgiveness, but He is also just.