The entire request lacks a protocol specification, so the server will treat it as a HTTP/0.9 request for a resource named /HTTP/1.1.

Only if properly implemented. And, to be specific, the assumption wouldn't be that it was an "HTTP/0.9 request", but a simple-request which was defined in the original HTTP/1.0 RFC. Servers are supposed to respond to a simple-request with a simple-response but, in the real world, many servers respond to simple-requests with full-responses (which early 0.9 clients would not understand.)

The important point in understanding the OP's problem is that the server defaults to assuming that the browser is speaking a version of HTTP where that request would be considered complete. Beyond that, this discussion has now become academic and rife with useless pedantry, much of which would only matter to those of us who were dealing with these niggling details in the early-mid 90s.