If the algorithms for sorting and filtering and discovering aren't open source, a social app is not open. It really is that simple. Some open source artifact on your device wrapping proprietary code on a server is not in the category of "open source products" because the only parts that matter are entirely closed. "My HTML front end is licensed MIT" does not make your product which is entirely dependent on secret sau…
With all due respect, it sounds like you are not familiar with Bluesky.
As far as I know, the only proprietary code is the single Discover feed, which every user can un-pin from their feeds. My assumption for this being proprietary is so that it cannot be gamed. It could be for a darker reason, I suppose.
However, this is a feed that I use maybe 10% of the time. Aside from the Following feed, I have curated a bunch of my own user lists and 3rd party feeds. I am able to pin these to my feeds list. This is where I spend my time.
Aside from that one significant feed, yet small aspect, literally every other part of Bluesky is open source. This includes client, server, moderation system, and protocol.