Comment by darrenf

They're in the HTTP/1.0 spec. https://www.rfc-editor.org/rfc/rfc1945#section-10.15

10.15 User-Agent

   The User-Agent request-header field contains information about the
   user agent originating the request. This is for statistical purposes,
   the tracing of protocol violations, and automated recognition of user
   agents for the sake of tailoring responses to avoid particular user
   agent limitations.