Existing Wording
Using the Condition resource to record an assertion that a patient doesn't have a particular diagnosis, etc.

#2: This means that implementations are not inherently required to "support" a modifier extension in any meaningful way - they may achieve this understanding by rejecting instances that contain this extension. Applications may also be able to ignore a modifier extension if that can know that this is safe to do in its own context, though this would not usually be the case.

Comments
This and several of the other examples in this section are legitimate clinical assertions that need to be made at times. There has been a huge amount of discussion (in CCDA) about how to handle negation (like "no known allergies" or "rule out diagnoses") and unknown data, and an urgent need for examples. So I think it is important for FHIR to specify not just "avoid modifier extensions where possible" but to offer positive guidance on how to represent these cases, which have already proven to be problemmatic in interoperability. While FHIR suggests possible examples to handle it, they are not normative. There needs to be a way for a creator of a FHIR resource to be able to express negation in a standardized way, and for receivers to be required to understand such negation.

Grahame's Comments
make comments about the specific resources. This is describing the general fall back position, and so the statement as provided stands. Note, though, that it's getting harder to find good easy to understand examples of modifier extensions as the specification fills out