The FHIR package registry is the npm-style ecosystem for Implementation Guides. Packages are tarballs of an IG's conformance resources, semver-versioned (hl7.fhir.us.davinci-pas#2.1.0). You can query and download for free; publishing typically goes via an RSS feed plus a PR to github.com/FHIR/ig-registry, or through Simplifier.

Useful API surface today without a key: search (GET https://packages.simplifier.net/catalog?name=davinci), version list (…/<package-name>), and tarball download (…/<package-name>/<version>). The UI at registry.fhir.org and the HL7 mirror at packages2.fhir.org are the same ecosystem from different doors.

What ImOnFHIR does with it: registry search + package-list.json is the whole ingestion story for keeping our IG stack current. We resolve and validate against published packages; we do not host licensed vocabularies that may appear inside those packages (CPT and similar stay point-don't-host).