XCB_CONN_CLOSED_EXT_NOTSUPPORTED should be removed
In Xlib, applications can make calls to extension API functions without breaking the connection object. In particular, a common pattern is to call QueryVersion and use the error return of BadRequest to indicate that the extension is not supported by the target X server.
XCB should allow the same behavior, at least with the query_version request, to avoid applications which don't check for the extension causing the connection to be disabled. Returning zero from the request function will cause the reply function to fail, returning NULL, which can be easily detected by the client.