Why isn’t “interpretation of the slope in context” in the 3.9 standard? (3.9)

The reason is that the relationship may be non-linear (see Explanatory note 4).

If a linear model is appropriate then interpreting the slope in context is one of the features that could be described.  I suggest that interpreting the slope in context when a linear model is appropriate is included in the teaching of this material.  One intention of this standard is that the bullet points in Explanatory note 3 apply to linear and non-linear relationships.