Was the ethical requirement to discuss and acknowledge predecessors work as developed in Newton's time as in our own? I would naturally expect not because it would seem to me to be the kind of thing that develops over time, but I could be wrong as I am not a historian of science.
It's a not a requirement, it's just a natural to do as a research scientist and a student of knowledge to acknowledge prior contributions if you knew it. The same is true when you are at the time of Aristotle.
John Maynard Keynes is famously quoted as stating: