I noticed if you use the search functionality on an iOS device you can still access old deleted iMessages.
In order to reproduce, delete an iMessage. Then go to the search screen by tapping home button twice. Search a word in that deleted iMessage. You will see the message in the search results.
In order to reproduce, delete an iMessage. Then go to the search screen by tapping home button twice. Search a word in that deleted iMessage. You will see the message in the search results.
This works even for messages deleted 1 month ago.