Client can only send this message during onboarding period. It is only valid if client previously sent C2S::JOIN message during the onboarding. If not, server considers that as client mistake. After successfully leaving the game, client may rejoin again with C2S::JOIN message. Clients who left the onboarding won't be playing the game and won't be able to join the game after game starts.
struct c2s_leave_t {
char type; // MSG_C2S_LEAVE
};