Retrieving deleted messages
How do I retrieve deleted messages
iPhone XS, iOS 12
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If you're using iCloud Syncing for Messages, messages are not part of your back up. Once you delete a message it's gone for good.
If you're not using iCloud Syncing and you have a back up that contains the messages, you can restore your phone from that back up:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Note that restoring your phone from a backup will overwrite everything currently on the phone.