add_action('init', 'registerCustomTaxonomies');
function registerCustomTaxonomies() {
    register_taxonomy(
        'products',
        'product', [
            'label' => 'Product Categories',
            'has_archive' => false,
            'hierarchical' => true,
            'rewrite' => [
                'slug' => 'products',
            ],
        ]
    );
}

add_action('init', 'addTaxonomiesToPages');
function addTaxonomiesToPages() {
    register_taxonomy_for_object_type('products', 'product');
}

flush_rewrite_rules();