FluentCartExcludeProductByCategory.php

<?
add_filter('fluent_cart/shop_query', function ($query, $params) {
    if (is_admin()) {
        return $query;
    }

    // Term IDs to exclude (product-categories)
    $excluded_ids = [11];

    $query->whereDoesntHave('wp_terms', function ($rel) use ($excluded_ids) {
        $rel->whereHas('taxonomy', function ($tt) use ($excluded_ids) {
            $tt->where('taxonomy', 'product-categories')
               ->whereIn('term_id', $excluded_ids);
        });
    });

    return $query;
}, 10, 2);

add_filter('get_terms', 'exclude_fluentcart_categories_frontend_only', 10, 3);
function exclude_fluentcart_categories_frontend_only($terms, $taxonomies, $args) {

    if (!in_array('product-categories', $taxonomies)) {
        return $terms;
    }
    
    if (is_admin()) {
        return $terms;
    }
    
    if (is_singular('fluent-products')) {
        return $terms;
    }
    
    $is_frontend_shop = (
        !wp_doing_ajax() || 
        (isset($_REQUEST['action']) && $_REQUEST['action'] === 'fluent_cart_shop_products')
    );
    
    if ($is_frontend_shop || !wp_doing_ajax()) {
        $excluded_category_ids = array(2); //category ID
        
        if (!empty($terms) && is_array($terms)) {
            $terms = array_filter($terms, function($term) use ($excluded_category_ids) {
                return !in_array($term->term_id, $excluded_category_ids);
            });
        }
    }
    
    return $terms;
}