Is the Web Messaging Guest session duration related to Threading Timeline?

No, the messaging threading timeline does not affect Web Messaging Guest session duration. Threading timeline is limited to the internal conversation model. If threading timeline is set to zero, then the internal conversation is ended when the agent disconnects. If the end-user sends a new message over the same Guest session that will generate a brand new conversation ID, which in turn would trigger new flow execution and new transcript for Agent, while the end-user’s experience is not affected. End-user can continue the existing conversation, as long as the Guest session has not expired. For more information, see Messaging threadline timeline.