The great scientist Johannes Kepler wrote a very unusual work called Somnium ("The Dream"). It is sort of a cross between a science fiction story and a scientific description of the Moon.
Somnium began as a student dissertation in which Kepler defended the Copernican doctrine of the motion of the Earth, suggesting that an observer on the Moon would find the planet's movements as clearly visible as the Moon's activity is to the Earth's inhabitants. Nearly 20 years later, Kepler added the dream framework, and after another decade, he drafted a series of explanatory notes reflecting upon his turbulent career and the stages of his intellectual development. The book was edited by Ludwig Kepler and Jacob Bartsch, after Kepler's death in 1630.
https://en.wikipedia.org/wiki/Somnium_(novel)#Development
Somnium was first published in 1634. And no doubt it described how (though not why) the Moon was tidally locked to the Earth. In fact it gives different names to the two hemispheres of the Moon.
Levania is divided into two hemispheres called Privolva and Subvolva. The two hemispheres are divided by the divisor. Privolva never sees Earth (Volva), Subvolva sees Volva as their moon. Volva goes throughout the same phases as the actual Moon.
https://en.wikipedia.org/wiki/Somnium_(novel)#Plot_summary
So as early as 1634 readers interested in science could find a description of the near side of the Moon that always sees the Earth and the far side of the Moon that never sees the Earth.
And I expect that any previous scientists and philosophers who wrote about the moon would have noticed that the pattern of light and dark areas - or at least as much of it as was in sunlight at the moment - was always the same, and thus that the Moon always kept the same side facing the Earth. And that would prove that there was a side of the Moon that never faced the Earth.
So I expect that philosophers and scientists knew that there is a far side of the Moon which always faces away from Earth for thousands of years.
Added Dec. 06 2021.
According to The Guinness Book of Astronomy Facts & Feats, Patrick Moore,1979, 1983, page 26:
The first detailed statement that the Moon has a synchronous rotation was made by G.D. Cassini in1693, though Galileo may also have realized it.
So Moore was unaware of any scientist or philipsopher saying that one side of the moon always faced the Earth, and the other always faced wawy, before Cassini in 1693. That means that Moore didn't know of any pre telescopic statement that one side of the Moon always faced the Earth. And Moore might have been correct about pre telescopic observations.
But Kepler's Somnium does state that one side of the Moon faces the Earth and one side faces away from the Earth, and thus is the far side.
