The Interface Specification has been moved to the dfinity/portal repository.
The relevant files include:
docs/references/http-gateway-protocol-spec.md
docs/references/ic-interface-spec.md
docs/references/_attachments/certificates.cddl
docs/references/_attachments/http-gateway.did
docs/references/_attachments/ic.did
docs/references/_attachments/interface-spec-changelog.md
docs/references/_attachments/requests.cddl