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Oh, I wrote this from my mobile phone.

Now I'm at the computer and I'm giving you the example with multiple categories.

You can only leave one category there.

{block name='product_flags'}
    <ul class="product-flags js-product-flags">
        {if $page.page_name == 'category' && isset($category['id']) && in_array($category.id, ['3', '138'])}
            <li class="product-flag new">{l s='discountinued' d='Shop.Theme.Catalog'}</li>
        {else if in_array($page.page_name,['product', 'index']) && in_array($product.id_category_default, ['3', '138'])}
            <li class="product-flag new">{l s='discountinued' d='Shop.Theme.Catalog'}</li>
        {/if}
        {foreach from=$product.flags item=flag}
            <li class="product-flag {$flag.type}">{$flag.label}</li>
        {/foreach}
    </ul>
{/block}

In Home:

In category:

In product detail: