This is manifestly a gloss inserted in the older narrative by the later editor of the sacred text, to explain the use of the term in 1 Samuel 9:11, 1 Samuel 9:18-19. It is one among many instances which prove how the very letter of the contemporary narratives was preserved by those who in later times compiled the histories. We cannot say exactly when the term “seer” became obsolete. See the marginal references.
Related Commentaries of 1 Samuel 9:9
1 Samuel 9:9
9 (Beforetime in Israel, when a man went to enquire of God, thus he spake, Come, and let us go to the seer: for he that is now called a Prophet was beforetime called a Seer.)