This short Slate Explainer article notes that it is said that Constantine outlawed crucifixion after his conversion, although the matter is not without doubt. In any event, it is interesting to be reminded about the nastiness of the punishment (and to be reminded that Saudi Arabia still does it - even in this last week):
Even if Constantine did, in fact, end the practice of crucifixion, it’s not clear that he did so out of respect for Christ’s execution. Aurelius Victor, the earliest historian to claim that Constantine banned crucifixion, explained that the emperor was motivated by a sense of humanity rather than piety. Crucifixion is a pretty gruesome way to go—significantly worse than the New Testament makes it seem. Although Christ reportedly expired in a matter of hours, many crucifixion victims clung to life for days. Even in Roman times, it was considered an exceptionally cruel punishment, reserved mainly for those who challenged state authority, such as insurgents and enemy soldiers. (Joel Marcus of Duke described crucifixion as “parodic exaltation,” because it gave rebels the fame they sought, albeit in a grotesque form.) By some accounts, Constantine replaced crucifixion with hanging, a less painful execution method. Constantine’s supposed ban on crucifixion came as part of a package of reforms, further suggesting that he was merely exercising human mercy. Branding prisoners’ faces, for example, was also prohibited around the same time—a reform that had nothing to do with Christ’s execution.
Whether or not Constantine put a stop to Roman crucifixions, he definitely kicked off the Christian fascination with crucifixion and the cross. Before Constantine’s reign, it appears that images of the crucifixion were mainly used by pagans to taunt Christians. The third century Alaxamenos graffito depicts a worshipper standing next to a donkey-headed man on a crucifix. The inscription reads, “Alexamenos worships god.” Not until the fifth century did Christians widely adopt the crucifixion as their own symbol, and the faithful then sought out pieces of Christ’s cross.