I can take a guess why it's not there: The user should have the belief that all of their messages are being saved indefinitely. A false sense of security can gi

I can take a guess why it's not there: The user should have the belief that all of their messages are being saved indefinitely. A false sense of security can give rise to the behaviour of sharing too much. There is no disappearing message feature which actually prevents the message from being saved in some form: 1. Many apps with this feature still allow screenshots of the one-time or time-limited message. 2. The apps which block or notify of screenshots, are trivially defeated by using a second device to record the screen. 3. Jail-breaks exist specifically to disable screenshot protection and notification features. Finally you can set your own messages to auto delete, ans this is the most likely route of probing your message history. (I also understand that the default setting is to delete all messages after 30 days.)