Back-in-stock notify link

When a variant is sold out, swaps the buy button for an email-me link that pre-fills the product name.

{%- assign variant = product.selected_or_first_available_variant -%}
{%- unless variant.available -%}
  {%- assign subject = 'Notify me when back in stock: ' | append: product.title -%}
  <div class="bis">
    <p class="bis__msg">This item is currently sold out.</p>
    <a class="bis__link"
       href="mailto:hello@yourstore.com?subject={{ subject | url_encode }}&body={{ product.url | prepend: request.origin | url_encode }}">
      Email me when it's back
    </a>
  </div>
{%- endunless -%}
<style>
  .bis{margin:12px 0}
  .bis__msg{font-weight:600;margin:0 0 8px}
  .bis__link{display:inline-block;padding:12px 20px;border:1px solid #111;border-radius:6px;
    text-decoration:none;color:#111;font-weight:600}
</style>

Captures demand you would otherwise lose the moment something sells out, with zero app dependency. Paste into main-product.liquid next to (or replacing) the add-to-cart button.

Notes

Replace hello@yourstore.com with the inbox you actually monitor for restock requests.
The link only appears when the selected variant is genuinely unavailable, so it never contradicts a buyable product.
For multi-variant products, re-evaluate on variant change so a sold-out size shows the link while in-stock sizes show the buy button.