A timechain is needed for identity management, but not for the social network itself. Nostr is not an identity manager.
Discussion
Why is a timechain needed for identity management?
In Nostr, if an identity leaks, the holder abandons the key and adopts a new one. Even if we don’t want to think about it now, in the end every type of identifier is only attached to us for a given period of time. So, to manage it, a notion of time is needed.
Is this like when Shakespeare wrote "what's in a name" except it's "what's in a key"?