wordpress-tag_slug__not_in.php

<?php

// Add tag_slug__not_in to valid taxonomy queries
add_action('parse_tax_query', function($wp_query) {
	$q = &$wp_query->query_vars;
	$tax_query = array();

	if (!empty($q['tag_slug__not_in'])) {
		$q['tag_slug__not_in'] = array_map('sanitize_title_for_query', array_unique((array)$q['tag_slug__not_in']));
		$tax_query[] = array(
			'taxonomy' => 'post_tag',
			'terms' => $q['tag_slug__not_in'],
			'field' => 'slug',
			'operator' => 'NOT IN'
		);
	}

	// Merge with existing $wp_query->tax_query
	if (!empty($tax_query)) {
		$tmp_tax_query = new WP_Tax_Query($tax_query);
		$wp_query->tax_query->queries = array_merge(
			$wp_query->tax_query->queries,
			$tmp_tax_query->queries
		);
		$wp_query->tax_query->queried_terms = array_merge(
			$wp_query->tax_query->queried_terms,
			$tmp_tax_query->queried_terms
		);
	}
});