Perhaps Lobsters? https://lobste.rs/about#invitations
>The full user tree is public and each user's profile shows who invited them. This provides some degree of accountability and helps identify voting rings.
For a single community like Lobsters, you don't need the digital signatures part at all.
Keybase https://keybase.io/ has a feature to help you aggregate different identities/accounts, though I'm not sure how active it is after their acquisition by Zoom in 2020.
While I see the appeal of this kind of system, it also could act to silo communities towards groupthink, exacerbating the misinformation woes that have fractured our society. "Trust, but verify" would be an interesting mantra to try to encode into such a system to resolve that.
you don't need any keys for trust, you can imagine a centralized system and "trust" can be expessed as "adding person to your friends list". Then you the social network recommends you posts of the friends of your friends
what I'm trying to say, I think you don't need public keys to organize content filtering
and to answer the titular question: I don't know such a community. maybe it's possible to use an existing social network and add extra javascript on top to filter posts based on your "web of trust"?