@Eomm Yeah, I can probably do that. (update the docs)

So, the current PR implementation does not break anything, and undefined still defaults to 'request-id'.
In my opinion this is unexpected behaviour. I think it should be false by default and you should opt-in rather than opt-out.

Also, setting requestIdHeader to true instead of a string will result, in the same behaviour as false because
false | string is expected, should this be handled specifically or is it the users fault?

PS: here is why:

 const fastify = Fastify({
    requestIdHeader: true,
  })

const requestIdHeader = (options.requestIdHeader === false) ? false : (options.requestIdHeader || defaultInitOptions.requestIdHeader) 
// => requestIdHeader = true

...
const id = requestIdHeader === false ? genReqId(req) : (req.headers[requestIdHeader] || genReqId(req))
// req.headers[true] is undefined, so genReqId(req) will be used

sorry, didn't mean to do that. I updated the docs

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