See Rabbi David Zvi Hoffmann.

It is explicit in Ibn Ezra. About such additions he writes: "and the enlightened one will remain silent at that time."

How can one say that this is not from Moses? How does this fit with the words of Maimonides in his introduction to Perek Helek? According to Maimonides, is someone who says these verses are a later addition a heretic?

"The eighth principle: that the Torah is from Heaven. That is, we should believe that this entire Torah that was given through Moses our teacher, peace be upon him, is entirely from the mouth of the Almighty. Meaning that it all reached him from God, may He be blessed, in a manner metaphorically called 'speech.' And it is not known how it reached him, except that Moses, peace be upon him, was like a scribe to whom one dictates and he writes down all the events of the days, the stories, and the commandments. Therefore he is called 'lawgiver.' And there is no difference between 'And the sons of Ham: Cush and Egypt,' 'and his wife's name was Mehetabel,' 'and Timna was a concubine,' and 'I am the Lord your God' and 'Hear, O Israel.' For all of it is from the mouth of the Almighty, and all of it is the Torah of the Lord, perfect, pure, holy, and true.

And one who says that regarding some of these verses and stories, Moses told them on his own initiative—such a person is, in the eyes of our sages and prophets, a heretic, and reveals the face of the Torah improperly more than all other heretics. For he thinks that the Torah has core and shell, and that these historical accounts and stories are of no benefit, and that they are from Moses our teacher, peace be upon him.

And this is the meaning of 'the Torah is not from Heaven' (Sanhedrin 90). The sages said: this refers to one who believes that the whole Torah is from the mouth of the Almighty except for this verse, which the Holy One, blessed be He, did not say, but Moses said on his own. And this is 'for he has despised the word of the Lord.'
Blessed and exalted be God above the words of the heretics.
Rather, every single utterance in the Torah contains wisdoms and wonders for one who understands them, and the full extent of their wisdom has not been attained; it is longer than the earth in measure and broader than the sea. A person has only to follow in the footsteps of David, the anointed of the God of Jacob, who prayed: 'Open my eyes, that I may behold wondrous things out of Your Torah' (Psalms 119)."

First, it is clear that this is the accepted view, and still there are considerations of common sense and logic. And if that is my factual conclusion, then that is what I think. Whether they label me a heretic is completely irrelevant to the question of what the truth is. See Column 69.

Second, as was mentioned here, there are also medieval authorities (Rishonim) who accept later additions. Ibn Ezra was mentioned here, and there is more in Miriam Weitman's article in Megadim, and also the Tzioni and his students (despite Rabbi Moshe Feinstein's declarations).

I relate to the statement that the entire text is from Sinai as a normative declaration (that one should treat all of it as though it was given at Sinai), not a historical-factual one. Like the rabbinic expressions about everything that an experienced student will one day innovate, and its general principles and details being from Sinai, and the like.

With God's help, 27 Kislev 5782

To Noam—greetings,

Ibn Ezra responds sharply to the words of a commentator who claimed that the section of the eight kings was written in the days of Jehoshaphat, and explains that "before any king reigned" means before Moses, about whom it is said in the Torah: "And he became king in Jeshurun, when the heads of the people gathered together, the tribes of Israel as one."

Best regards, Elyam Fishel Workheimer

See here at length (especially note 3): https://www.knowingfaith.co.il/%D7%9E%D7%A7%D7%A8%D7%90-%D7%95%D7%90%D7%A8%D7%9B%D7%99%D7%90%D7%95%D7%9C%D7%95%D7%92%D7%99%D7%94/%D7%A4%D7%A1%D7%95%D7%A7%D7%99%D7%9D-%D7%9E%D7%90%D7%95%D7%97%D7%A8%D7%99%D7%9D-%D7%91%D7%AA%D7%95%D7%A8%D7%94#_edn3