wp-rocket-disable-async-css.php

/* Disable WP Rocket option 'async_css'. */
add_action( 'wp', function () {

    // Stop here if WPR is not active.
    if ( ! function_exists( 'get_rocket_option' ) ) {
        return false;
    }

    // Only on front-end.
    if ( ! is_admin() ) {

        // Pass 0 as value to WPR option 'async_css'.
        add_filter(
            'pre_get_rocket_option_async_css', 
            '__return_zero'
        );
    }
});