This gist will create a persistent WP admin notice.

Remote WP Admin Notice

add_filter( 'admin_notices', 'my_remote_admin_notice' );
add_filter( 'network_admin_notices', 'my_remote_admin_notice' );

function my_remote_admin_notice() {

    $notice_url = 'INSERT_URL_HERE';
    $message = json_decode( wp_remote_retrieve_body( wp_remote_get( $notice_url ) ), true)['content']['rendered'];

    if( is_wp_error( $message ) ) {
        return false;
    }
    ?>
    <div class="notice notice-success is-dismissible admin-notice-remote">
        <div class="notice-content">
            <?php echo $message; ?>
        </div>

        <button type="button" class="notice-dismiss">
            <span class="screen-reader-text">Dismiss this notice.</span>
        </button>
    </div>
    <?php
}