505 HTTP Version Not Supported
I do not speak that protocol version — the rarest refusal of the five houses.
The server refuses the major protocol version the request declares — an HTTP/3.0 reaching an implementation that only speaks 1.1, or an archaeological HTTP/0.9. RFC 9110 suggests explaining in the body which languages the house speaks.
In practice it is nearly impossible to provoke: new versions negotiate over ALPN before HTTP begins, and servers speak everything clients use. When a real 505 shows up, the usual suspect is a malformed request with a corrupted version line.
HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain
Connection: close
This server speaks HTTP/1.1 only.