how to recover an imessage thread

Hi there.

Unfortunately, the only way to recover a deleted iMessage threat is to restore a previous iPhone backup that contains that thread. If you have iCloud backups and you are willing to reset your iPhone before restoring a previous backup, see this Support link: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Let me know if you need any more assistance.

If you are using iCloud syncing for messages, once you delete a message, it's gone from all of your devices. There are no back ups.