I basically lived in Wire shark for a couple of years working for a voip company and still use stuff like curl all the time and I don't think I could walk through an http request of the top of my head.
CGI is also cool to learn about the workings of, since it almost seems too simple.
Sure I did.
Then I forgot most of the details because they didn't matter, and I knew I could look them up quickly if I ever needed to write a HTTP client/server for some reason.
Because you did: after that you have to provide a Host: <hostname>.
You need two newlines to finis the request, plus the HTTP 1.1 standard requires clients to send a Host: header for all requests.
Not saying every interviewer would care about that in an early screening process.
Yeah expecting many people to be able write out a complete http request from memory without a reference to look at. But the general structure of a http request is something so basic to web development that asking what the structure of a http request looks like isn't an unreasonable expectation.
Request line (method, uri), header(s), empty, body...