CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTP09_ALLOWED, long allowed);
A HTTP/0.9 response is a server response entirely without headers and only a body. You can connect to lots of random TCP services and still get a response that curl might consider to be HTTP/0.9!
Since 7.66.0, libcurl requires this option set to 1L to allow HTTP/0.9 responses.
CURL *curl = curl_easy_init();
if(curl) {
CURLcode ret;
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
curl_easy_setopt(curl, CURLOPT_HTTP09_ALLOWED, 1L);
ret = curl_easy_perform(curl);
}