Who made the decision on this? Where is the “smoking gun?”
The people who are responsible for introducing nomina sacra into the text are likely the same ones who discontinued using the divine name.
When I mentioned the quality of the Leviticus fragment with Yaho, what I had in mind was Peitersma’s comment that “the Septuagintal credentials of [the manuscript] are well nigh impeccable”. I quote that from memory and I think that’s the exact phrase he used.
More importantly, if Christians were using the Tetragrammaton to call upon the God of Israel, it would have caused massive controversy.
There is a lot of evidence that ordinary use of the divine name in the form Yaho was widespread among ordinary Jews in the first century. See the many sources that indicate this in Frank Shaw’s book The Earliest Non-Mystical Jewish Use of IAW (2014).
Was It the Christians? If Christians systematically removed the Tetragrammaton, why did they replace it with Kyrios—a term already used for Jesus throughout the NT? This substitution only reinforces the NT’s high Christology, where Jesus is identified with the Lord of the OT (e.g., Romans 10:13 citing Joel 2:32).
Yes, because their theology had moved on from the monotheism of the NT toward a Trinitarian reading that incorporated Jesus in a triune God. Howard and Trobisch both explain this factor as important in the disappearance of the divine name from Christian texts. Here is Howard’s explanation:
Is there any way for us, at this late date, to calculate the effect which this change in the Bible had on the second century church? It is of course impossible to know with certainty, but the effect must have been significant. First, a number of passages must have taken on an ambiguity which the original lacked. For example, the second century church read, “The Lord said to my Lord” (Matthew 22:44, Mark 12:36, Luke 20:42), a reading which is as ambiguous as it is imprecise. The first century church probably read, “YHWH said to my Lord.”
To the second century church, “Prepare the way of the Lord” (Mark 1:3) must have meant one thing, since it immediately followed the words: “The beginning of the gospel of Jesus Christ.” But to the First Century Church it must have meant something else since they read, “Prepare the way of YHWH.”
The second century church read 1 Corinthians 1:31, “The one who boasts, let him boast in the Lord,” which was probably considered a reference to Christ mentioned in verse 30. But to the first century church, it probably referred to God mentioned in verse 29 since they read, “The one who boasts let him boast in YHWH.”
These examples are sufficient to suggest that the removal of the Tetragrammaton from the New Testament and its replacement with the surrogates kyrios and theos blurred the original distinction between the Lord God and the Lord Christ, and in many passages made it impossible to tell which one was meant. This is supported by the fact that in a number of places where Old Testament quotations are cited, there is a confusion in the manuscript tradition whether to read God or Christ in the discussion surrounding the quotation. Once the Tetragrammaton was removed and replaced by the surrogate “Lord”, scribes were unsure whether “Lord” meant God or Christ. As time went on, these two figures were brought into even closer unity until it was often impossible to distinguish between them. Thus it may be that the removal of the Tetragrammaton contributed significantly to the later Christological and Trinitarian debates which plagued the church of the early Christian centuries.
See the rest of his article here.
https://library.biblicalarchaeology.org/article/the-name-of-god-in-the-new-testament/
Trobisch argues similarly in his book, focussing on 2 Cor 3.14–18 as his example, and Luise Schottroff focuses on 1 Cor 2.16 to make a similar point.