I can't find the "Leave Silently" option.
"Leave Silently" is available when you update KakaoTalk to v.3.5.0 or a higher version.

Please update your KakaoTalk app to the latest version and access Kakao Lab again.

[How to enable "Leave Silently"]
• KakaoTalk> Settings (⚙️) > Kakao Lab > Leave Silently ON/OFF

▣ Important
• This feature cannot be used in private chats and Open Chat.
• You cannot choose "Leave Silently" if you leave a chatroom from the chat list.