It is not only stated in the Revelation, although in the judgment of almost all historians who are not Preterists, that was the last book of the Bible to be written. It is also stated in the following
11 According to the sentence of the law in which they instruct you, according to the judgment which they tell you, you shall do; you shall not turn aside
to the right hand or
to the left from the sentence which they pronounce upon you.
Deuteronomy 17:11
And in the words,
5 Every word of God
is pure; He
is a shield to those who put their trust in Him. 6 Do not add to His words, Lest He rebuke you, and you be found a liar.
Proverbs 30:5-6
Yes, but now read it correctly.
In Deuteronomy, God is referring to the Law - the Torah, of which Deuteronomy is the final piece. The entire Bible, all that would come later, is not the Torah, and most of what remains in the Old Testament is not God speaking directly. No more law is given by God in the Old Testament. Do not add to the Law is what God said in Deuteronomy, not "do not add to the Bible". The Bible and the Law are not at all synonymous. It is adding to the Bible to state they are.
The admonition to add nothing nor subtract anything from this scroll, in Revelation, is a direct reference to THAT scroll - Revelation - which John was told to take up and write. It does not in any sense apply to the rest of the Bible, which was not compiled until the 300s AD.
As to the admonition in Proverbs - absolutely so, but what are the words of God? The words of the Law, the rTorah, the words that God spoke. Solomon's proverb there is not words spoken by God, but by Solomon (et al). To call them "God's words" when they are Solomon's words is to add to the words of God. All of the LAW in Scripture came out of God's mouth. Jesus told the Devil that man lived on every word "that proceeds forth out of the mouth of God" - a lengthy and rather convoluted expression. The Bible always identifies the words that God spoke himself. It is THOSE words that Jesus referred to, specifically - the words that proceeded forth out of the mouth of God - that Jesus referred to.
The Torah is all law that came directly out of God's mouth. Jesus' words are recorded in quadruplicate.
THOSE are the words that God spoke, that can't be added to. To take the whole Bible and claim that God spoke all of that is not true. The Bible itself makes a very clean and clear distinction of what God said. And the Bible says - in those passages you quoted - to focus on what God said. To take, for example, what Jude said in his letter about Enoch and claim that Jude's words were spoken by God is to mash a bunch of words that God did not speak with what he did speak - and to break the commandment. Jesus gave commandments. If he wasn't really God, then what he did was blasphemous. That's why he was killed: the Sanhedrin did not believe he was divine.
The tradition of holding up the entire Bible and pretending that God spoke all of that is in violent conflict with these very passages warning people not to do that. It is a stubborn tradition, and one that we will not be ending today. Nevertheless, no: God did not speak out the whole Bible. Nor did he arrange the Bible, deciding what was in and what was out. Men did that. IN the Bible, what God said is recorded, mostly in the Torah, a few of the prophets, the Gospels and Revelation. The rest of the Bible is not what God said. It's what men said about God, and that does not have the same authority.
To say that it does is to add massively to the words of God.