This prophecy has been fulfilled in the sense that the Bible has been preached among all the peoples of the earth. The record of the Old Testament was never lost to mankind. However, Lehi's version of the plates of brass was more complete than our Old Testament, and we are told that the lost portions of the Old Testament are yet to be revealed.
Bruce R. McConkie
"In this connection, be it also remembered that the brass plates that Nephi took from Jerusalem contain more of the word of the Lord for the comparable period than does our present Old Testament. They, of course, will also come forth in due time as part of the restoration of all things. Indeed, Lehi prophesied 'that these plates of brass should go forth unto all nations, kindreds, tongues, and people who were of his seed. Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time.' ("1 Ne. 5:18"1 Ne. 5:191 Ne. 5:18-19.)" (Bruce R. McConkie, The Millennial Messiah: The Second Coming of the Son of Man, p. 113.)
Orson Pratt
"These plates of brass, contained the prophecies of all the holy Prophets from the beginning—from the days of Adam; hence they must have contained the prophecies of Enoch, Adam, Noah, Abraham, Isaac, Jacob, and Joseph in Egypt. The prophecies of Isaiah and many others of the holy Prophets were contained upon these plates of brass.
J863"Notwithstanding brass is a metallic substance capable of being dissolved and crumbling back, in a few years to the elements, yet there was a miracle wrought upon these plates of brass. The Prophet said that these plates of brass should not be dimmed by time, that God would preserve them to the latest generations. What for? In order that they might come forth and their contents be translated by the Urim and Thummim, that these contents might be declared to all nations, and kindreds, and tongues, and people, who were the descendants of Lehi upon the face of all this continent, from the frozen regions of the north to the very utmost extremities of South America. That all these nations should come to a knowledge of the things contained on those plates of brass." (Journal of Discourses,16: 55)