>and support searches for unavailable (out-of-catalog) entities

This is a subtle killer feature of Netflix's search, and something not often done by others. By including these unavailable entities, it allows Netflix's similarity and recommendation engines to still understand what the user was looking for, and provide things that are relevant and similar, even if they aren't exactly what the user is looking for.

That being said, this is a lot easier for something that has a relatively finite, known set of items (movies) in contrast to things with lots of items that can be differentiated in smaller, subtler ways (e-commerce, web documents).