There are definitely solutions for handling this with consistency semantics that are something weaker than linearizable consistency. But they usually become expensive in other ways and loose a lot of the availability benefits to recover the consistency properties you do need. Most of the models we're even talking about here are something in the realm of sequential or causal consistency, which are still considered strong consistency models (and are usually implemented on top of linearizable algorithms that allow for weakening invariants that protect against witnessing inconsistency. spanner is an example of this).

Your friend-state solution only protects the visibility of the post if the friend-state between the two users has not changed more recently than both the post and your version.

I.E.

Friend state 0 is we're friends.

Friend state 1 is we're not friends.

Friend state 2 is we're friends.

Friend state 3 is we're not friends.

I have friend state 2, and the post is at state 1. I accept my version incorrectly and am able to view the post, despite the fact that we're no longer friends.