WordPress REST API access only to logged in users.

gistfile1.txt

/* ------------------------------------------------------------------------- *
 *  Returning an authentication error if a user who is not logged in tries to query the REST API
/* ------------------------------------------------------------------------- */
function only_allow_logged_in_rest_access( $access ) {
    if( ! is_user_logged_in() ) {
        return new WP_Error( 'rest_API_cannot_access', 'Only authenticated users can access the REST API.', array( 'status' => rest_authorization_required_code() ) );
    }
    return $access;
}
add_filter( 'rest_authentication_errors', 'only_allow_logged_in_rest_access' );
 
remove_action( 'wp_head', 'rest_output_link_wp_head', 10 );
remove_action( 'template_redirect', 'rest_output_link_header', 11 );
remove_action( 'xmlrpc_rsd_apis', 'rest_output_rsd' );