Earlier this year, Whatsapp made an announcement that it would encrypt all users data but a researcher differs on that.
The researcher, Zdziarki said that he found traces of Whatsapp deleted chats on Whatsapp version on iOS which can be merge back to form a conversation even if the they were cleared or archived.
Speaking from his blog, Zdziarski said that Apple's iMessage app also suffered from the same problem and the only way to truly dispose the chats is to delete Whatsapp from your device. Still, Zdziarski urged users not to panic but they should be aware of law enforcement agents who might try to muscle their way to access peoples privacy. But Zdziarski was also concerned about some other implications . He said,
" Software authors should be sensitive to forensic trace in their coding.
The design choices they make when developing a secure messaging app has
critical implications for journalists, political dissenters, those in
countries that don’t respect free speech, and many others. A poor design
choice could quite realistically result in innocent people – sometimes
people crucial to liberty – being imprisoned".