Most often you need to erase private data when you are selling old iOS device. However, if you delete files by using delete keys, the files still resides at the same storage location, it just is simply flagged or marked as deleted and still can be recoverable.
One way to prevent nosy people from finding your deleted iMessages is to keep them from gaining access to your iPhone in the first place. Or permanently erase them.
Safrai browsers store a list of the private pages you visit on your iPhone, iPad and iPod device in your browsing history. This article will help you remove them.