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As I understand it, you are saying if you override the undocumented (but publicly-named) default_request_version attribute, HTTP 0.9 requests no longer work.
I suspect it is even broken by default. My understanding is with HTTP 0.9 you should be able to send b"GET <path>\r\n" and get a response, but Python’s server will deadlock waiting for a second blank line or EOF. It looks like this deadlock has been there since ~forever (1995).
See Issue 10721 which already proposes to remove 0.9 server support. I would be weakly in favour of this (or more strongly if someone can prove my theory that the current support is broken).