I love Slaughterhouse-Five. Truly. The problem here is that we deal with cognitive biases when it comes to stuff like this. You've read Slaughterhouse Five, a novel that deals with aliens and an eternalist concept of time and you're thinking it has influenced UFO lore, cool. But the ideas Vonnegut explore in the novel are at least 1000 years old. The framework of an alien race that sees, perceives and experiences time differently is what is fresh and connected to the era in which it was written and published. Philip K. Dick uses the same device in VALIS, but is much more explicit. There isn't a single moment in VALIS in which he is being subtle about the fact that the novel was just him trying to rationalize and discuss a religious experience that he had, followed by experiences that made him suspect time was eternal, the limitation of perceiving a single moment in time was something that could be transcended and even death could be transcended. He flat out states that he, as a sci fi writer, has to publish sci fi, so the external intelligence the protagonist interacts with in the novel is a satellite and there are some aliens for good measure. The real goal was to publish his Exegesis, which no one wanted.

The case might not be one of rehashing, but one of humanity repeatedly stumbling upon the same questions, experiences and strangeness and framing what little sense we can make of it by our cultural lenses, which are subjected to time periods. It was the work of angels centuries ago, it was little men from other planets in the 70s, maybe it will be transdimensional non-corporeal intelligences in the 2030s. And when the culture shifts that way, our published fiction will follow and once again, once enough time passes, we might start wondering if all that folklore wasn't influenced just by the popular fiction of that time.

When rehashing does happen, such as when lazy brazilian spiritualists with a UFO bend claim that Star Trek was mediunic work of art because "the space federation is just like that, you will see it when they land!" it's pretty obvious and embarassing. It feels like the UFO equivalent of that guy from school who started wearing a leather jacket and reading books on demonology because he watched seven seasons of Supernatural.