I think there needs to be a subsection in the specification which describes required and permitted installer behaviors. Maybe there is one and I missed it, but --no-default-extras is currently only mentioned in the rejected ideas section.
Given a package pkg1 with a dependency on pkg2, are installers allowed to omit pkg2’s default extras?
I don’t think the answer is a strict yes or no based on my interpretation of the document. But in spite of the nuance, we should strive for clarity. “SHOULD install the default extras”, “MAY implement a user facing mechanism to opt out”, etc.