If humans build a world from scratch, say a colony on another planet, or a generation ship such as in this question, can we ask how their ethics would affect their choices in how to design such a world?
I definitely think ethics are a major part of worldbuilding and should be on topic when asking how a particular set of ethics could affect the world. Now that I look further at your question, I think you should have to define the particular ethics of your world. There are so many ethical systems and minor points that I don't think asking, is this ethical, is clear enough. You need to specify the ethical system of the society in question before we can answer your question. Is lying ethical to your colonists? Is murdering disendants? We need further clarification in your case to answer, but the world consequences of ethics should be just as much on topic as the effects of magic or science. Just like in those questions, you need to give some guidenance on what you mean by your key term.
It seems to me that ethical questions are perfectly reasonable, but the writer seems to want an "objective" answer. That's not possible: ethics is intrinsically subjective. This is not to say that ethics is always simply a matter of opinion, but rather that there is no agreement as to any absolute or stable point from which to extrapolate an objective ethics.
Thus, I suggest that clarification or editing insist on either
- a specific proposal as regards a base ethics system within which to address the question, or
- a question which makes such a proposal the desired result.