"I, the Lord, have sworn that I will never be angry with you and I will never rebuke you. For the mountains may be removed and the hills may shake, but My loving kindness will not be removed from you, and My covenant of peace will not be shaken," says the Lord who has compassion on you. (Isaiah 54:9-10)
Here are two scriptures that confirm that this is indeed a new covenant promise. They show us that God's "covenant of peace" mentioned in Isaiah 54 is the new covenant of the blood of Christ's cross which makes peace between us and God.
"This is My blood of the covenant, which is poured out for many for forgiveness of sins" (Matthew 26:28).
"Christ made peace with us through the blood of His cross" (Colossians 1:20).
The meaning of angry in Isiah 54:9
God will never be angry with you also means: He will not be displeased with you. He will not be offended with you. He will not be upset with you. He will not be exasperated with you. He will not have wrath for you. He will not resent you.
The meaning of rebuke in Isiah 54:9
God will never rebuke you means: He will not repel you from Himself. He will not beat you back. He will not shame, reject, disgrace, blame, disapprove of or condemn you. He will not reject you as worthless. He will prove you to be worthy. He will not break you. He will not mark you as immoral. He will not ruin you. He will not see you as unclean.
It does not have the same meaning as the word rebuke in Proverbs 3:11-12 or in Hebrews 12:5, "Do not lose heart when the Lord rebukes you." There it means "to correct, to lead you out of false beliefs."