WooCommerce: Custom dropdown for variation selection

I think the condition here has_term($term_slug, $taxonomy, $variation->get_id()) won’t work for variation. If you want to check term, we need to check with parent variable product instead of checking variation because Woocommerce doesn’t store any taxonomy/term attribute to variation.

Instead of using has_term, we can use your existing attributes in get_variation_attributes to get and check condition because a variable product may have defined taxonomy as attribute or custom attribute defined directly in Attributes tab. As you can see in my screenshot below:

So I updated your code to make the data-variation-id work properly

if($product->is_type('variable')) :
        $available_variations = $product->get_children();
        
        if($attributes = $product->get_variation_attributes()){
            foreach($attributes as $attribute_name => $options) :
                $taxonomy        = str_replace('attribute_', '', $attribute_name);
                $attribute_label = wc_attribute_label($taxonomy);
                echo '<label for="' . esc_attr($taxonomy) . '">' . esc_html($attribute_label) . '</label>';
                echo '<select name="' . esc_attr($taxonomy) . '" id="' . esc_attr($taxonomy) . '">';
                
                $variation_ids_map = [];
                
                foreach($options as $option_slug) :
                    $current_variation_id = '';
                    $term                 = get_term_by('slug', $option_slug, $taxonomy);
                    $term_name            = $term ? $term->name : $option_slug; // for custom attribute
                    
                    foreach($available_variations as $variation_id_candidate) :
                        $variation = wc_get_product($variation_id_candidate);
                        if($variation && $variation->attributes[$taxonomy] == $option_slug) :
                            $current_variation_id = $variation->get_id();
                            break;
                        endif;
                    endforeach;
                    
                    $variation_ids_map[$option_slug] = $current_variation_id;
                    
                    echo '<option value="' . esc_attr($option_slug) . '" data-variation-id="' . esc_attr($current_variation_id) . '">' . esc_html($term_name) . '</option>';
                
                endforeach;
                
                echo '</select>';
            
            endforeach;
        }
    endif;