The mystery Paul spoke of in Romans 11:11 is not a new revelation as some has indicated it has always been veiled prophecy (both near an far)
Paul says in Romans 11:11.
Again I ask: Did they stumble so as to fall beyond recovery? Not at all! Rather, because of their transgression/false step, salvation has come to the Gentiles to make Israel envious.
Paul as a schooled Pharisee would have known the song by Moses in Deuteronomy 32:21, Romans 10:20
Deuteronomy 32:21
They made me jealous by what is no god and angered me with their worthless idols. I will make them envious by those who are not a people; I will make them angry by a nation that has no understanding.
Paul is applying this prophecy to the current situation, where salvation has come to the Gentiles, making Israel envious, resulting in the inclusion of the Gentiles in Gods plan of salvation. He starts this explanation in the previous chapters of Romans and builds on it.
Veiled or hidden does not mean it is not there, rather to use an analogy of a veil. A bride is still there, under the veil, she is concealed until the veil is lifted. Just as the veil is used hide the face of the bride, God has "hidden" His plan for humanity as they are ready to receive it. He unveils it, as He wills. This doesn't mean God deliberately hid the truth, but rather, He chose to reveal it in a way that aligns with His sovereign plan, that is both merciful and just. It was not hidden by God, but in God. Revealed in the Person of Christ.
Because of the hardness of the hearts, his people could not comprehend that God's purposes were greater than their immediate situations, even if the prophets all hinted at it.
In a similiar way they could not see God in the person of Jesus because of that same hardness and spiritual blindness. They could not perceive the Promised Seed. They could not comprehend the prophesied King.
What is the transgression/the false step that they took? Unbelief, they did not believe in the Messiah that stood in their midst.
Romans 16:25-27
Now to him who is able to establish you in accordance with my gospel, the message I proclaim about Jesus Christ, in keeping with the revelation of the mystery hidden for long ages past, but now revealed and made known through the prophetic writings by the command of the eternal God, so that all the Gentiles might come to the obedience that comes from faith— to the only wise God be glory forever through Jesus Christ! Amen.
It was always the same good news. Paul did not bring a separate message of eternal salvation to Gentiles.