It seems to me that now SO tries to only have questions that accept clear, definitive answers. Is this true?
Yes.
Is there another place in the SO network for this kind of questions?
Not really.
However, the question you actually ask:
what makes a REST API documentation stand out?
While subjective to a certain degree, is a better question because it's asking for how you evaluate something. As such it's better than asking which is best as this would just get you a list of people's current favourites which is both highly subjective and too localised to the time you ask the question. Learning how to evaluate something is a skill that will stand the test of time.
As to where this question could go - Programmers seems like the best bet. Just tell them I sent you :)