Is it not reasonable that Umar ibn al-Khattab did not know that the recitation of the verse of stoning (ayah al-rajm) had been abrogated while its ruling remained, especially since he wanted to write it in the Mus'haf were it not for his fear of people saying that he had added to the Book of Allah?

Umar ibn al-Khattab, may Allah be pleased with him, knew that the verse of stoning (rajm) was abrogated in recitation. Therefore, he refrained from writing it in the Mus'haf for fear that it would be considered an addition to the Qur'an. Umar's statement, "Were it not that people would say, 'Umar has added to the Book of Allah'," was a way of emphasizing that he did not add to the Book of Allah, not out of fear of people. Nevertheless, Umar did not want to write the verse within the main text of the Mus'haf; rather, he intended to write a note in the margin of the Mus'haf indicating that the Prophet, peace and blessings be upon him, and the Companions had carried out stoning. He did not do so for fear that something not belonging to the Book of Allah might be added to it, or that people would say he had added to the Qur'an. This demonstrates his extreme diligence in preserving the Qur'an in its pure form. The Book of Allah is preserved by the preservation of Allah the Almighty.