Function joined_since 

async fn joined_since(
    services: &Services,
    room_id: &RoomId,
    user_id: &UserId,
    since: u64,
) -> bool

Whether a user’s join to a room is fresh since a sync token.

The joined count moves on every join and forced state replay except a local user’s profile change over the position its join recorded, which keeps it. An unreadable count reads as unmoved.