Godzilla: King of the Monsters implies that humans and Titans peacefully coexisted thousands of years ago, but the credits suggest that in actuality, humans and Titans have always been enemies. Apparently, mankind has a very long and complex history with the Titans in the MonsterVerse.
After Godzilla is seemingly killed by the Oxygen Destroyer in Godzilla: King of the Monsters, Monarch goes looking for him deep in the ocean, where they find the Hollow Earth tunnels. Down below, they discover the ruins of an ancient underwater civilization which may or may not be the mythical city of Atlantis. According to Dr. Chen (Zhang Ziyi), the underwater city is older than both the Romans and the Egyptians. They're able to learn from the murals on the temple walls that in this place, humans actually worshipped Titans as gods and revered them. Here, it appears that they were treated with respect rather than fear.
However, the redacted text in the credits of Godzilla: King of the Monsters sheds light on the real relationship between ancient humans and Titans. This text tells a story that goes back thousands of years, back when Titans were still being worshipped. It seems that it didn't take long for this relationship to change. Humans once depended on Titans for protection, but they grew to see them as weapons. Humans believed that they could use Titans for purposes of warfare, and devised methods of telepathic communication. This evolved into a way to control them. The Titans rebelled against their would-be masters, and a war erupted.
The outcome of the war was that the civilization scattered and fell apart. The fighting triggered an unspecified "cataclysm" which in turn created an Ice Age, and it took quite a while for mankind to recover. What all this means is that the relationship between humans and Titans hasn't changed that much over the years. King of the Monsters creates a nice picture of a better time where humans accepted Titans, but the credits contradict that image by revealing that the people who worshipped them tried to use them as weapons.
There was an idea in King of the Monsters that humans don't view Titans now in the way that they did before, but the credits exposes a startling truth about humanity in the MonsterVerse. Thousands of years may have passed, but ancient humans didn't treat Titans any better than the government that wanted to kill them in the beginning of the movie.
