You need to differentiate between citing everything, and citing a select few prior works which are integral to your own. For the latter, it is very natural to mention and cite them, unless you have motive not to do so.
So it seems you have established that no, Newton was not expected to cite everyone in his time, very well, this now puts the question further up the chain over my entry into the conversation - "Were the people Newton neglected to cite actually integral to his work?"
My definition of integral would be, two possible definitions:
1. do I write something here that someone reading it would say, "What?! What do you mean, explain yourself, how did you arrive at this surprising bit of information?!!" then it is integral.
2. Am I doing this based on some work that someone did without which I would definitely not be doing this? If I am doing it with knowledge of the prior work but whether or not the prior work existed I would still be doing it, the prior work is not actually integral.
Newton was accusing Leibniz plagiarising his work on calculus and engaged in a bitter, lifelong priority dispute over the invention of calculus.