If any of these deprecated features is a cause for concern for you, please email the curl-library mailing list as soon as possible and explain to us why this is a problem for you and how your use case can't be satisfied properly using a work around.
Supporting this is non-obvious and might even come as a surprise to some users. Potentially even being a security risk in some cases.
curl 7.64.0 introduces options to disable/enable support for this protocol version. The default remains supported for now.
The support for HTTP/0.9 will be switched to disabled by default in 6 months, in the September 2019 release (possibly called curl 7.68.0).