IMO the idea of a truly decentralized recommendation system is misguided. Decentralization has limits, Nostr shows this and any system which is just a client talking to relays, or decentralized in some other way, is limited.

Luckily, what one can have is a FLOSS recommendation system. Any instance can fetch posts from Nostr relays and show results. Of course, you can't prove for sure the authenticity of the code running on the server, as a user, but it would still be better than centralized platforms.

A feature of Nostr is how easy fetching data is. I think it's great for applications like this.