Jamieson, Fausset and Brown on Jeremiah 50:20

20. The specification of "Israel," as well as Judah, shows the reference is to times yet to come.

iniquity ... none--not merely idolatry, which ceased among the Jews ever since the Babylonian captivity, but chiefly their rejection of Messiah. As in a cancelled debt, it shall be as if it had never been; God, for Christ's sake, shall treat them as innocent (Jr 31:34). Without cleansing away of sin, remission of punishment would be neither to the honor of God nor to the highest interests of the elect.

whom I reserve--the elect "remnant" (Is 1:9). The "residue" (Zec 14:2; 13:8, 9).