deleted iMessages aren’t gone
Make note of the fact that even after a text message is deleted on an iPhone it is still temporarily stored on the device. An iPhone uses a database to track texts. A text the user deletes will be checked off as such in the database. The message is removed from the device later. Deleted messages are purged as necessary to in order to free up space on the phone with older messages being removed first.
It is not unusual for a message deleted more than a year earlier to be recoverable.
Different service providers will retain messages on their servers for different periods, but usually no more than two weeks.