Re: 505 response a MUST?

>>  ... and section 3.1 spells out various rules about version number
>>  usage, but does not specify that a server MUST send a 505 response
>>  if it receives a major version number higher than the highest
>>  version it implements.

That isn't why 505 was created.  It allows a future server to deny
service to older protocols.  Since we cannot know whether HTTP/2.0
is incompatible with HTTP/1.1 (the major version change does not imply
incompatibility, it just removes the requirement for compatibility),
it is inappropriate for a server to be required to respond in error
to a message it might be able to respond to normally.  That's why it
is not a MUST, and why Apache responds normally to an HTTP-version of
HTTP/2.0 if it can interpret the request as an HTTP/1.1 server.

....Roy