As far as I see the constant is still there (in the most recent 5.3.2.x version).
However, I think a better way of changing the default stock level is decorating the \XLite\Model\Product::getDefaultAmount() method, not AMOUNT_DEFAULT_INV_TRACK constant itself.
Something like this:
PHP Code:
// File: classes/XLite/Module/DEV_ID/MODULE_ID/Model/Product.php
namespace XLite\Module\DEV_ID\MODULE_ID\Model;
class Product extends \XLite\Model\Product implements \XLite\Base\IDecorator
{
/**
* Default qty value to show to customers
*
* @return integer
*/
public function getDefaultAmount()
{
return 1;
}
}