REQUIRED: The theme must not used the tags. | REQUIRED: The theme must not call to wp_title()

WordPress added support for the title-tag feature in version 4.1 and it’s now a required feature for themes uploaded to the repo.

To implement this feature, make sure your theme does not have the title tag hard coded within header.php, e.g.:

<title><?php wp_title( '|', true, 'right' ); ?></title>

Configure your theme with title-tag support like this:

add_action( 'after_setup_theme', 'wpse_theme_setup' );
function wpse_theme_setup() {
    /*
     * Let WordPress manage the document title.
     * By adding theme support, we declare that this theme does not use a
     * hard-coded <title> tag in the document head, and expect WordPress to
     * provide it for us.
     */
    add_theme_support( 'title-tag' );
}

To make changes to the title text, use the following filters (source):

• pre_get_document_title short-circuits wp_get_document_title() if it
  returns anything other than an empty value.

• document_title_separator filters the separator between title parts.

• document_title_parts filters the parts that make up the document
  title, passed in an associative array.