Every published FHIR Implementation Guide is supposed to maintain a package-list.json at its canonical URL: a machine-readable version history with package-id, canonical, and a list[] of versions. Each entry carries version, path, status (ci-build, draft, ballot, trial-use, normative, corrected, withdrawn), sequence, fhirversion, date, plus a corrections[] array for technical corrections.

That file is how an IG catalog stays current without scraping HTML. For any IG we care about (PAS, DTR, CRD, PDex, US Core), fetch <canonical>/package-list.json and you know every version, its status, and whether the package you validate against is still the current published one.

What ImOnFHIR does with it: we use package-list as the feed for the validator's IG catalog freshness story — "you're validating against X; current published is Y." Point-don't-host: we sync metadata and package references; we don't republish HL7's IG content as our own.