Disable XML-RPC via snippet

From the xmlrpc_enabled filter docs:

Contrary to the way it’s named, this filter does not control whether XML-RPC is fully enabled, rather, it only controls whether XML-RPC methods requiring authentication – such as for publishing purposes – are enabled.

To fully disable XMLRPC, you might be able to use the xmlrpc_methods filter and return an empty array:

add_filter( 'xmlrpc_methods', 'wpse416991_disable_xmlrpc' ); 
/**
 * Disables XMLRPC methods.
 *
 * @param  array $methods The XMLRPC methods.
 * @return array          An empty array to disable all methods.
 */
function wpse416991_disable_xmlrpc( $methods ) {
    return array();
}

Per @birgire’s comment, you could further simplify this using __return_empty_array():

add_filter( 'xmlrpc_methods', '__return_empty_array' );

If that doesn’t work, you can block access to the XMLRPC file in your .htaccess file (though that’s outside of the WordPress scope):

<Files "xmlrpc.php">
    Require all denied
</Files>