Remove sold-out variants. Only works for products that have one option. It won't work with products that have two or three options, like Size and Color.

remove-sold-out.liquid

{% if product.options.size == 1 %}
  <script>
  var $addToCartForm = $('form[action="/cart/add"]');
  if (window.MutationObserver && $addToCartForm.length) {
    if (typeof observer === 'object' && typeof observer.disconnect === 'function') {
      observer.disconnect();
    }
    var config = { childList: true, subtree: true };
    var observer = new MutationObserver(function() { 
      {% for variant in product.variants %}
        {% unless variant.available %}
          jQuery('.single-option-selector option').filter(function() { return jQuery(this).text() === {{ variant.title | json }}; }).remove();
        {% endunless %}
      {% endfor %}
      jQuery('.single-option-selector').trigger('change');
      observer.disconnect();
    });  
    observer.observe($addToCartForm[0], config);
  }
  </script>
{% endif %}