Source: Harold W. Attridge, "Josephus and His Works," Jewish Writings of the Second Temple Period, ed. Michael E. Stone (Philadelphia: Fortress, 1984) 223.
"Josephus maintains that the 'exact succession of the prophets', which guarantees the historical reliability of the records which they composed, ended in the Persian period (Ag. Ap. 1:41), but he is equally convinced that the phenomenon of prophecy continues and bears constant testimony to divine providence. From the post-exilic and Hasmonean periods he cites the evidence of the prophetic dreams of Alexander the Great and the high priest Jaddus (Ant. 11:327, 333). Later he discusses the prophetic gifts of John Hyrcanus (13:282, 300, 322) and of the Essenes (13:311). In the Herodian period prophecy continues, and, not surprisingly, bears out the evaluation of Herod discussed in Josephus' editorial comments. Perhaps the most important example is the prophecy of the Essene Manaemus (Menahem) to the boy Herod. The prophet predicts Herod's elevation to royalty and advises the lad 'to love justice and piety toward God and mildness toward your citizens,' but he predicts that Herod will not do so: 'you will forget piety and justice. This, however, cannot escape the notice of God, and at the close of your life His wrath will show you that he is mindful of these things' (15:375-6)."
I'm not sure if Josephus believes that the canon has been completely closed. He says that the biblical history is authoritative because there was a continuous succession of prophets at the time of its composition, which is not true of subsequent histories. But does he believe that such a succession can return in the future? He doesn't say, at least not in Against Apion 1:41. As far as he's concerned, however, the canon is closed for the time being.
In debates about Christian cessationism--the idea that spiritual gifts like tongues, prophecy, and miracles ceased with the completion of the New Testament--the cessationist side acts as if charismatics want to add to the Bible. After all, the charismatics affirm that God continues to give new revelations, do they not? And, if God speaks to individual Christians, then that is God speaking, so why wouldn't his words be as authoritative as Scripture? If the charismatics are correct, then we'd might as well as Rick Joyner's words to the canon, right?
But many charismatics would answer "no" to that question, for they view Scripture as authoritative in a unique sense. And, apparently, that was Josephus' stance as well: he believed that God could still communicate to his people through prophecy, since people need continual guidance from God, even after the Scriptures have been completed. At the same time, God has revealed sacred history, which contains the foundational lessons that Israel needs to learn. And that's the only infallible record of the past, as far as Josephus is concerned.