I'm glad to see this is being addressed. I'd like to advocate against defaulting to SNOMED's "family history of X" concepts at all and simply stick to the latter format ('family history with explicit context' + value_as_concept_id).
This subset of SNOMED concepts is much less comprehensive than we have available in the condition domain. While the "family history of X" concepts are hierarchical (though I don't see the relationships in ATHENA), the number of concepts are far fewer.
To stick with the same example, say your source data contains:
"Family history of asthma" has no child concepts where "Asthma" has over a hundred descendants. You could either lose information by mapping all three to "family history of asthma", or use the proposed "when available" approach which would prevent you from leveraging the concept hierarchy:
From an ETL and cohort definition perspective this seems unnecessarily complicated when compared to:
Additionally, if we are strictly using this format we can specify the relation of the relative with a more specific observation concept, e.g. "family history with explicit context pertaining to father".