I would like to add some of my further research which aims to answer the question: "Is it possible to draw a conclusion of when (mayta) first was prohibited in the Quran?".
As @Seeker said in his answer, I came to the conclusion that the first prohibition in the Quran of dead meat (mayta) was in either of the verses:
- Al-An'aam (6:145)
- An-Nahl (16:115)
In my research I found out that it seems that Surat An-Ana'am was revealed in the end of the Meccan period, probably the last year before the Hijra while Surat An-Nahl says to be revealed some months before the Hijra. This should then give that al-An'aam was revealed before. See "Muhammad Asad The Message of the Quran":
WITH the possible exception of two or three verses, the whole of this surah was revealed in one
piece, towards the close of the Mecca period - almost certainly in the last year before the
Prophet's exodus to Medina.
Muhammad Asad The Message of the Quran, Al-An'am (Cattle)
ACCORDING to almost all the authorities (including the Itqan), this Surah was revealed a few
months before the Prophet's emigration to Medina. Although some commentators maintain that
the last three verses belong to the Medina period, there is no evidence for this more or less
Muhammad Asad The Message of the Quran, An-Nahl (The Bee)
The verse in Surat al-Ana'am helps us to understand that it was revealed before An-Nahl:
وَعَلَى الَّذِينَ هَادُوا حَرَّمْنَا كُلَّ ذِي ظُفُرٍ ۖ وَمِنَ الْبَقَرِ وَالْغَنَمِ حَرَّمْنَا عَلَيْهِمْ شُحُومَهُمَا إِلَّا مَا حَمَلَتْ ظُهُورُهُمَا أَوِ الْحَوَايَا أَوْ مَا اخْتَلَطَ بِعَظْمٍ ۚ ذَٰلِكَ جَزَيْنَاهُم بِبَغْيِهِمْ ۖ وَإِنَّا لَصَادِقُونَ
And to those who are Jews We prohibited every animal of uncloven hoof; and of the cattle and the sheep We prohibited to them their fat, except what adheres to their backs or the entrails or what is joined with bone. [By] that We repaid them for their injustice. And indeed, We are truthful.
Surat Al-Ana'am, verse 146
We notice that this is one verse after the mentioned 6:145.
While in Surat An-Nahl it says:
وَعَلَى الَّذِينَ هَادُوا حَرَّمْنَا مَا قَصَصْنَا عَلَيْكَ مِن قَبْلُ ۖ وَمَا ظَلَمْنَاهُمْ وَلَٰكِن كَانُوا أَنفُسَهُمْ يَظْلِمُونَ
And to those who are Jews We have prohibited that which We related to you before. And We did not wrong them [thereby], but they were wronging themselves.
The key here is "which We related to you before", i.e at least in Surat Al-An'aam.
The first verse in the Quran which talks about mayta being prohibited should be in Surat Al-An'aam verse 145, this according to above comments.
This said, I still do not know when exactly mayta (dead meat) was prohibited the first time, because in the same Surat, some verses earlier it is said that the Muslims already has got a good explanation of what was forbidden to eat and what not:
وَمَا لَكُمْ أَلَّا تَأْكُلُوا مِمَّا ذُكِرَ اسْمُ اللَّهِ عَلَيْهِ وَقَدْ فَصَّلَ لَكُم مَّا حَرَّمَ عَلَيْكُمْ إِلَّا مَا اضْطُرِرْتُمْ إِلَيْهِ
And why should you not eat of that upon which the name of Allah has been mentioned while He has explained in detail to you what He has forbidden you, excepting that to which you are compelled
Surat Al-Ana'am, verse 119
This would then indicate that the Prophet (saw) had instructed the believers (by wahy which wasn't revelead in the Quran) of what to eat (and not to eat) before any verse was revealed.