I am currently trying to understand the details of the German law about defects ("Sachmangel, § 434 BGB") and misleading advertising ("Gesetz gegen den unlauteren Wettbewerb (UWG), § 5 Irreführende geschäftliche Handlungen") and how they are related to the law in other countries such as US and UK. Let's consider the following example:
You can buy a lot of products that can be extended with specifically tailored accessories. For example, there are specific adapters available for connecting a given smartphone with a display or cars can be equipped with matching roof racks.
The availability of given accessories can very well influence the decision to buy something so traders advertise their products accordingly, such as "For this product, the accessory [...] is available to enable functionality [...]".
Now let's imagine you buy the product, but you notice very soon that it is not possible to buy the accessory anywhere (for the sake of argument there is also no third party manufacturer that provides a matching accessory). It might be the case that this accessory was never manufactured at all or it is out-of-stock and will never be manufactured again. The true reason is of course not known to the consumer.
How does this relate to the status of the main product? I can imagine three different reasonings.
If the car was advertised with an A/C, but it is broken, you can not use the A/C functionality and thus it is a defect of the car and you can expect repairs or (monetary) compensation. Correspondingly, if the advertised roof rack is not available, you can not use the "putting stuff on the roof" functionality that was promised in the advertisement, thus it is a defect of the main product, too.
If an ad shows a smartphone used in a swimming pool, but it is not waterproof, this is misleading advertisement. While the individual consumer can not do anything about it, authorities can perform certain actions against it. Correspondingly, if there is an ad where a smartphone is connected to a display, but it is not possible for anyone else to reproduce this setup, this is misleading advertisement.
Promising the availability of accessories is a purely informative statement and has no legal consequences. This, however, would enable manufacturers to just promise and advertise certain functionality without putting any engineering effort into the actual implementation. Edit: Of course, this would be noticed fast for very popular products, but in some areas such as household appliances, the variety of models is large enough that a public outcry is unlikely.