Wasnt there an rfc recommending against the use of X- prefix headers? I guess that didnt hold
`X-AMZN-something` at least though probably predates that; a mix of `X-AMZN` and `AMZN` would be annoying. (Although I suppose they could say 'We've deprecated `X-`, it'll be stripped and still work, but please don't use it in new code' applying to all of them, old and new.)