Isn't this the concept behind BlueSkye? https://docs.bsky.app/docs/starter-templates/custom-feeds

Bluesky feeds are still server-side (due to needing to process all of the available posts to generate the feed) but at least you can choose which ones to use and people can make their own, which is an improvement over a single app-provided algorithmic feed.

How would do collaborative filtering while respecting privacy?

Working on it. :-)