Recover iMessage from iPhone
Iow can I recover iMessage thread I just accidentally deleted
iPhone 6 Plus, iOS 12
Question marked as
Top-ranking reply
Assuming you don't house your messages in iCloud, your messages are saved on an iPhone backup that was created while the messages existed. Restore that backup to retrieve the messages (and other data) present at the time the backup was created.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support