Many priests, bishops and even a couple of popes have claimed that the Church is against the death penalty for convicted criminals.
And in the political arena the death penalty is just about extinct except in places like North Korea, China and Islamic countries. In many countries where the death penalty is banned for criminals, politicians don't seem to have a problem with the death penalty for innocent babies, and in many cases the two opposites go together - life for criminals, death for babies.
Pope Francis, in Amoris Laetitia 83, says:
Similarly, the Church not only feels the urgency to assert the right to a natural death, without aggressive treatment and euthanasia”, but likewise “firmly rejects the death penalty”.
The synod which preceded Amoris Laetitia says much the same (Relatio Finalis 2014 para 64):
Similarly, the Church not only feels the urgency to assert the right to a natural death, without aggressive treatment and euthanasia, but also takes care of the elderly, protects people with special needs, assists the terminally ill, comforts the dying and firmly rejects the death penalty.
Now Relatio Finalis refers for its authority to the Catechism, para 2258:
"Human life is sacred because from its beginning it involves the creative action of God and it remains for ever in a special relationship with the Creator, who is its sole end. God alone is the Lord of life from its beginning until its end: no one can under any circumstance claim for himself the right directly to destroy an innocent human being."
It appears that no Church document, apart from those produced by Pope Francis, forbids the death penalty. Instead, they confirm that no one can take innocent life.
The Church has made its stance on the death penalty very clear as regards the taking of non-innocent life:
Gen. 9:63: “Whoever sheds the blood of man, by man shall his blood be shed; for God made man in his own image.”
Lev 20:1 The Lord said to Moses, 2 “Say to the Israelites: ‘Any Israelite or any foreigner residing in Israel who sacrifices any of his children to Molek is to be put to death. The members of the community are to stone him.
20:9 “‘Anyone who curses their father or mother is to be put to death.
20:10 “‘If a man commits adultery with another man’s wife—with the wife of his neighbour—both the adulterer and the adulteress are to be put to death.
20:11 “‘If a man has sexual relations with his father’s wife, he has dishonoured his father. Both the man and the woman are to be put to death; their blood will be on their own heads.
20:12 “‘If a man has sexual relations with his daughter-in-law, both of them are to be put to death.
There are many passages like this in Leviticus. Who will accuse God of being somehow in error? The same for Deuteronomy 13:20-21. Matt 15:4 and Mark 7:10 sentence men to death for cursing parents, so it's not confined to the Old Testament. Rom 13:4 and Heb 10:28 say quite clearly that authorities have the power of the death sentence.
There is no doubt that a Pope can bind the faithful by issuing a new regulation forbidding the death penalty for Catholics. It is not impossible that he could declare that from now on the Church rejects capital punishment, just as the Church changed the sabbath to Sunday, although given the evidence of God's clear approval of the death penalty for the guilty, this would take courage.
What he cannot do is issue a statement saying that the Church already rejects or has always rejected what God has decided and the Holy Spirit has verified for a number of circumstances and on many occasions.