Post Admin – Filter by posts without tags

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The first part is to add a dropdown using restrict_manage_posts filter, and the second one is to actually get posts without tags using pre_get_posts filter:

function wpse147471_add_no_tags_filter() {
    if ( 'post' !== get_current_screen()->post_type ) {
        return;
    }

    $selected = ( isset( $_GET['tag-filter'] ) && 'no-tags' === $_GET['tag-filter'] );
    ?>
    <select name="tag-filter">
        <option value="">&mdash; Select &mdash;</option>
        <option value="no-tags" <?php echo selected( $selected ); ?>>No Tags</option>
    </select>
    <?php
}
add_action( 'restrict_manage_posts', 'wpse147471_add_no_tags_filter' );

function wpse147471_get_posts_with_no_tags( $query ) {
    if ( ! is_admin() || ! $query->is_main_query() ) {
        return;
    }

    if ( ! isset( $_GET['tag-filter'] ) || 'no-tags' !== $_GET['tag-filter'] ) {
        return;
    }

    $tag_ids = get_terms( 'post_tag', array( 'fields' => 'ids' ) );

    $query->set( 'tax_query', array(
        array(
            'taxonomy' => 'post_tag',
            'field'    => 'id',
            'terms'    => $tag_ids,
            'operator' => 'NOT IN'
        )
    ) );
}
add_action( 'pre_get_posts', 'wpse147471_get_posts_with_no_tags' );

Looks like there is no cleaner way to get all posts without tags via WP_Query than using a 'NOT IN' taxonomy query with all existing tag IDs, but at least it works.

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