Audit stale health parameters and rating-area data exposed through metadata/API

[daphnehanse11]

Context

While reviewing health model cleanup candidates, a few objects appear to be unconsumed by current PolicyEngine-US formulas or data generation, but some are still exposed through metadata/API seed paths. We should audit them before deleting or renaming anything.

Candidates

Old IRS PTC parameter path

• gov.irs.credits.premium_tax_credit.eligibility
• gov.irs.credits.premium_tax_credit.phase_out.starting_rate
• gov.irs.credits.premium_tax_credit.phase_out.ending_rate

Current ACA formulas appear to use gov.aca.required_contribution_percentage.* and gov.aca.ptc_income_eligibility instead. Local searches did not find formula/data consumers for the old IRS path. However, policyengine-api-v2-alpha testing seeds still whitelist gov.irs.credits.premium_tax_credit, so this may be part of an API metadata contract.

ACA enrollment/spending state parameters

• gov.aca.enrollment.state
• gov.aca.spending.state

These are exposed in generated app metadata, but local searches did not find current formula/data consumers. We should decide whether these are intended calibration/reference targets, public API metadata, or stale parameters.

Medicaid geography table / hidden input follow-up

• policyengine_us/parameters/gov/hhs/medicaid/geography/second_lowest_silver_plan_cost.csv
• medicaid_rating_area

second_lowest_silver_plan_cost is imported but did not appear to be used by current formulas. medicaid_rating_area is exposed/seeded externally but appears to have no current model consumers. These may be old ACA/Medicaid geography artifacts and should be audited before cleanup.

Suggested resolution

1. Confirm whether any API partners depend on these names through metadata, reforms, or explicit inputs.
2. If no consumers exist, remove or deprecate them with a changelog entry.
3. If they are API contracts, mark them clearly as deprecated/compatibility-only and migrate consumers to the active ACA parameter path.
4. Consider adding a lightweight stale-parameter check that distinguishes internal formula usage from public API exposure.