What did the Old Testament practice of hanging a stoned criminal on a tree foreshadow?

The Old Testament practice of hanging a stoned criminal on a tree foreshadowed Christ's being made sin and accursed for us. It previewed what He accomplished at the cross, when God made Him to be sin for us, so the land would not be defiled and salvation could rest on His finished work.

Les explains that the law required a hanged criminal's body not to remain all night on the tree but to be buried that day, because he who is hanged on a tree is accursed of God, so the land would not be defiled. Israel would scramble to get him off the tree before sundown. Les says this awful practice was all a preview of what Christ accomplished in our behalf. He points to II Corinthians 5:21, "God hath made him to be sin for us." The rebel who was stoned to death and hanged on a tree as a public spectacle pictured the One who was to come, and it underlies teaching total salvation based on the finished work of the cross plus nothing.

Deuteronomy 21:22

And if a man have committed a sin worthy of death, and he be to be put to death, and thou hang him on a tree:

See the rest

What did the stoned and hanged rebel in the Old Testament picture?

The stoned and hanged rebel pictured Christ, who was hanged on a tree as accursed of God. That public spectacle was a preview of what Christ accomplished in our behalf, for God made Him to be sin for us. It pointed forward to the cross and His finished work.