Soorat al-Ahzaab was as long as Soorat al-Baqarah, then most of it was abrogated
Question

Theres a hadith in Musnad Ahmad that says 200 verses were abrogated from Surah Al-Ahzab. Is this true?
Praise be to Allah.Firstly: 
It was narrated by ‘Abdullah the son of Imam Ahmad in Zawaa’id al-Musnad (21207), ‘Abd ar-Razzaaq in al-Musannaf (599), Ibn Hibbaan in his Saheeh (4428), al-Haakim in al-Mustadrak (8068), al-Bayhaqi in as-Sunan (16911), Ibn Hazm in al-Muhalla (12/175), via ‘Aasim ibn Bahdalah, from Zirr, who said: Ubayy ibn Ka‘b said to me: How long is Soorat al-Ahzaab when you read it? Or how many verses do you think it is? I said to him: Seventy-three verses. He said: Only? There was a time when it was a long as Soorat al-Baqarah, and we read in it: “The old man and the old woman, if they commit zina, then stone them both, a punishment from Allah, and Allah is Almighty, Most Wise.” 
Ibn Hazm (may Allah have mercy on him) said:
This is a clearly saheeh isnaad, as clear as the sun, in which there is no fault. End quote. 
Ibn Katheer (may Allah have mercy on him) said: 
This is a hasan isnaad. This implies that there were more verses in it, then the wording and ruling were both abrogated. And Allah knows best.
End quote from Tafseer Ibn Katheer (6/335) 
There is a corroborating report that was narrated by ‘Abdullah ibn Ahmad in Zawaa’id al-Musnad (21206): Wahb ibn Baqiyyah told me: Khaalid ibn ‘Abdullah at-Tahhaan informed me, from Yazeed ibn Abi Ziyaad, from Zirr ibn Hubaysh, from Ubayy ibn Ka‘b, who said: How many (verses) do you recite in Soorat al-Ahzaab? He said: Seventy-odd verses. He said: We recited it with the Messenger of Allah (blessings and peace of Allah be upon him) like al-Baqarah, or more than that, and verily the verse of stoning was in it. 
Yazeed ibn Abi Ziyaad is da‘eef (weak), but there is nothing wrong with the hadeeth,as there is corroborating evidence for it. 
This indicates that Soorat al-Ahzaab was a lengthy soorah like Soorat al-Baqarah, but most of it was abrogated. 
In the answer to question no. 105746 we stated that abrogation in the case of the Quran is of three types: abrogation of both the verses and the ruling, abrogation of the ruling but not the verses, and abrogation of the verses but not the ruling. 
With regard to the abrogation that took place in Soorat al-Ahzaab of this large number of verses, some of it comes under the heading of abrogation of the verses but not the ruling, as in the case of the verse on stoning, and some of it comes under the heading of abrogation of both the verses and the ruling – as stated above in the words quoted from Ibn Katheer. 
Az-Zarqaani (may Allah have mercy on him) said:
With regard to abrogation of the verses but not the ruling, the fact that this occurred is indicated by the saheeh report from ‘Umar ibn al-Khattaab and Ubayy ibn Ka‘b, according to which they said: Among the words of Quran that were revealed were the words: “The old man and the old woman, if they commit zina, then stone them both”
You know that this verse no longer exists between the covers of the Mushaf or on the lips of the reciters, although the rulings remain in effect and have not been abrogated. 
The fact that this occurred is also indicated by the saheeh report from Ubayy ibn Ka‘b, according to which he said: Soorat al-Ahzaab was equivalent (in length) to Soorat al-Baqarah, or longer…
End quote. 
Secondly: 
With regard to what was mentioned in the report, that Ibn Mas‘ood (may Allah be pleased with him) used to say that the Mi‘wadhatayn (i.e., the last two soorahs of the Quran) were not part of the Quran, there are three scholarly views concerning that: 
-1-
That the report is not soundly narrated from him. Ibn Hazm (may Allah have mercy on him) said: 
Everything that has been narrated from Ibn Mas‘ood to suggest that the Mi‘wadhatayn and Umm al-Quran (the Essence of the Quran, i.e., Soorat al-Faatihah) were not part of the Mushaf, is false and fabricated, and is not saheeh. Rather what is narrated soundly from him is the recitation of ‘Aasim from Zirr ibn Hubaysh from Ibn Mas‘ood, which includes Umm al-Quran (al-Faatihah) and al-Mi‘wadhatayn.
End quote from al-Muhalla (1/32) 
An-Nawawi (may Allah have mercy on him) said:
The Muslims are unanimously agreed that the Mi‘wadhatayn, al-Faatihah and all the soorahs that are written in the Mushaf are Quran, and that whoever denies any of that has disbelieved. What has been narrated from Ibn Mas‘ood about al-Faatihah and the Mi‘wadhatayn is false and is not soundly narrated from him.
End quote from al-Majmoo‘ (3/396) 
-2- 
He only said that they were not written in the Mushaf; he did not deny that they are part of the Quran. 
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 
In Kitaab al-Intisaar, Al-Qaadi Abu Bakr al-Baaqillaani gave his interpretation of what was narrated from Ibn Mas‘ood, and he was followed in that by ‘Iyaad and others. He said: Ibn Mas‘ood did not deny that they (the Mi‘wadhatayn) were part of the Quran; rather he denied that they were written in the Mushaf. He thought that nothing should be written in the Mushaf except that which the Prophet (blessings and peace of Allah be upon him) gave permission to write in it. It is as if news did not reach him that he had given permission for that. This is another way of understanding the report on his part; it is not a denial that they are Quran. 
It is a good interpretation of the report; but the saheeh report that I quoted contradicts that view, as it says in it that he said that they are not part of the Book of Allah. Yes, the phrase ‘Book of Allah’ may be understood as referring to the Mushaf, thus it may be in harmony with the interpretation mentioned.
End quote from Fath al-Baari (8/743) 
-3-
Initially he used to deny they were part of the Quran, but when it became clear to him that they are part of the Quran, he wrote them in his Mushaf and recanted his earlier opinion. Az-Zarqaani (may Allah have mercy on him) said: 
It may be that the report that Ibn Mas‘ood denied that the Mi‘wadhatayn were part of the Quran – assuming that it is saheeh – was before he came to know of that, then when he came to know that they are part of the Quran, after that had been established by mutawaatir reports, and consensus had been formed that they were indeed part of the Quran, he was in the forefront of those who believed that they are part of the Quran. 
One of them said: It may be that Ibn Mas‘ood did not hear the Mi‘wadhatayn from the Prophet (blessings and peace of Allah be upon him), and he did not hear mutawaatir reports to that effect, so he refrained from accepting or rejecting them. He should not be denounced for that, because he was examining the issue and he had to be certain about it. Perhaps this response is the one that will put one’s mind at rest, because in the recitation of ‘Aasim from Ibn Mas‘ood the Mi‘wadhatayn are confirmed (as part of the Quran). This recitation is saheeh and was narrated soundly (via saheeh isnaads) from Ibn Mas‘ood. The report that Ibn Ma‘ood denied that the Mi‘wadhatayn were part of the Quran was also narrated via isnaads that were classed as saheeh by Ibn Hajar. Therefore this denial is to be understood as having been earlier in the life of Ibn Mas‘ood, so as to reconcile between the two reports.
End quote from Manaahil al-‘Irfaan (1/275-276) 
See also the answer to question no. 178209 .
And Allah knows best.