Viewing text messages
How can I view archived text messages?
iPhone 6s, iOS 11.0.3, Help
As there is no built in function to archive texts, the only way it could have happened is if you chose an archive method and completed it. I archive my texts using PhoneView (Mac). I save them as pdf files. Therefore, I view them with Preview on my Mac.
If you back up your phone, the texts are included in the back ups. However, in order to access them, you'd need to restore your phone from the back up. That also overwrites whatever is on the phone now.