Is it supererogatory to say "Ameen" after finishing the recitation of the concluding verses of Surat Al-Baqarah in prayer, and what is the evidence for that?

The 'ameen' (saying Amen) by the Imam at the conclusion of the supplication found at the end of Surat Al-Baqarah is legislated according to the majority of scholars. The questioner's statement that its legitimacy is exclusive to voluntary (nafl) prayers is a detailed opinion held by some scholars. Therefore, there is no blame on the Imam for what he did.