Re: X-Cart 5 and duplicate content...
 

In an ideal world, I think a product should have multiple settings for being disabled, something like so:

1) Unavailable for purchase

When this is selected, the product is still available by direct URL but does not show up in listings, search, or by any other way on the site except by direct URL. When accessed it shows a message that says the product is no longer available, removes purchase links, and recommends related products that are available. This allows the existing SEO rankings for the page to be maintained and prevents 404 errors. It could also integrate nicely with a notify me when back in stock module.

2) Disabled

The product 404's.

Re: X-Cart 5 and duplicate content...
 

That is perfect idea, Jon. Thank you so much for it!
I have created an idea topic out of it here:
http://ideas.x-cart.com/forums/229428-x-cart-5-x/suggestions/7502173-unavailable-for-purchase