CURLOPT_ALTSVC_CTRLoption. The bitmask allows the application to limit what HTTP versions to allow, and if the "cache" file on disk should only be used to read from (not write).
Alt-Svc:headers, libcurl will store those in the cache file at exit (unless the
CURLALTSVC_READONLYFILEbit was set).
Alt-Svc:is only trusted and parsed from servers when connected to over HTTPS.
Alt-Svc:headers is as of March 2022 still the only defined way to bootstrap a client and server into using HTTP/3. The server then hints to the client over HTTP/1 or HTTP/2 that it also is available over HTTP/3 and then curl can connect to it using HTTP/3 in the subsequent request if the alt-svc cache says so.