Function strip_prev_state 

fn strip_prev_state(
    pdu: PduEvent,
    sender_user: &UserId,
    in_timeline: impl Fn(&PduEvent) -> bool,
) -> PduEvent

State sections strip the stored prev_content/prev_sender pair (Synapse injects the pair on timeline fetches only). The requester’s own membership and events duplicated from the returned timeline (MSC4222, full_state) keep it: clients read membership transitions from those copies.