Jeremiah 29:11, "I know the plans I have for you," was part of a letter to whom?

Answer: The exiles in Babylon

Jeremiah sent the letter from Jerusalem to the elders, priests, prophets and people Nebuchadnezzar had carried into exile in Babylon.

“This is the text of the letter that Jeremiah the prophet sent from Jerusalem to the surviving elders among the exiles and to the priests, the prophets, and all the others Nebuchadnezzar had carried into exile from Jerusalem to Babylon.”— Jeremiah 29:1 (BSB)