Are there really sins reserved only to the pope to forgive?
There is a reason why some sins are reserved to the bishop or the Pope for forgiveness, and that’s because some of those sins incur canonical penalties and need to be lifted. There are like “crimes”, so they need to be forgiven by a bishop or the Pope or any priest they give special faculties to.
A bishop can lift most canonical penalties but some are reserved only to the Pope. However, in the danger of death, any ordained priest can validly lift virtually all penalties according to Canon 976:
“Even though a priest lacks the faculty to hear confessions, he absolves validly and licitly any penitents whatsoever in danger of death from any censures and sins, even if an approved priest is present”
So even a priest who lacks the faculty to hear confessions can absolve in this case because of the risk of death. The Church wants her children to make it to heaven without any impediments.
Each local bishop can choose to give some or all his priests faculties to lift some or all of the penalties following canonical crimes.
Now however, Pope Francis has extended the authority to all priests at confession to forgive the sin of abortion.:
“Given this need, lest any obstacle arise between the request for reconciliation and God’s forgiveness, I henceforth grant to all priests, in virtue of their ministry, the faculty to absolve those who have committed the sin of procured abortion. The provision I had made in this regard, limited to the duration of the Extraordinary Holy Year, is hereby extended, notwithstanding anything to the contrary. I wish to restate as firmly as I can that abortion is a grave sin, since it puts an end to an innocent life. In the same way, however, I can and must state that there is no sin that God’s mercy cannot reach and wipe away when it finds a repentant heart seeking to be reconciled with the Father. May every priest, therefore, be a guide, support and comfort to penitents on this journey of special reconciliation.” Pope Francis, Misericordia et misera (2016)