This is incorrect.
In Hebrews 1:2 it says God the Father made the world by the Son. In other words by the Spirit.
So there was a Son of God in the OT, He just wasn't a man; He was spirit.
It says in Hebrews 1:1 that in various ways God spoke in times past to the fathers, and then in verse 2 in the LAST DAYS by his Son. So those who claim a literal Son manifesting or speaking to anyone in the OT contradicts this verse.
I do see how someone could take verse 2 as being an eternal Son if they aren't committed to OT foundational truths about God, NT teachings about the Logos, and simple reality as to what a "Son" is. If we go the way of a literal eternal Son then we must dispatch with Luke 1:35 and sanity.
Hebrews 1:2 "but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world."
I believe the best explanation that harmonizes everything including verses like Luke 1:35 and Galatians 4:4 is that the one who became the Son created all things. We know from John 1 that through the Father's Logos, the worlds were created and then the Logos became flesh. It was this becoming flesh (a real Son) that is the reason he is called the Son, because that is what he actually was according to the angel and the reason why he is called the Son of God (Luke 1:35).
Historians could say that President Ronald Reagan was born in 1911. This is accurate and we understand the language to mean that the one who was born in 1911 was named Ronald Reagan and that later in his life he became President. We know the language doesn't mean he was President when he was born.
So too with the Son of God. The eternal Logos, who is God, became flesh and the one who is the Son is eternal, not as a literal Son, but as the eternal God Himself. We find confirmation of this in John 1 where it is only after verse 10 when the word is made flesh, that the term "Son" is used in this chapter. It doesn't say "In the beginning was the Son, and the Son was with God".
God is eternal and the Father's Logos was an ongoing manifestation of creation (Genesis 1) and ultimately culminating in the taking on of flesh (John 1:10). The Son is eternal in the sense that God becoming man was in God's plans as though it already was, but the literal Son in space and time is defined by Luke 1:35.
Some Trinitarians also get off track with the language of predestination as if God has done away with real free will and destined some to heaven and some to hell.