Comment:

Section 2.20.0.2 articulates safeguards for using modifier extensions – avoiding where possible, prohibiting processing of resources with unknown modifier extensions, and prohibiting use of modifiers within data elements. It seems that modifications should adopt these safeguards whether in extensions or not. E.g., absence of a condition or observation is easily indicated by an explicit element; embedding the modification within the element (e.g., in certain members of a value set) creates risk.

Summary:

Safeguards for modifier extensions should apply to all modifiers