Deleted messages

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Assuming you do not house your messages in iCloud, deleted messages are retained in your latest iPhone backup that occurred while the messages were present. Restore the 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

If you are using iCloud Syncing for Messages, once you delete them, they are gone.

As the previous poster noted, if you aren't using iCloud Syncing and you have a back up that contains the messages, you can restore your phone to that back up. Be aware, however, that restoring the phone overwrites everything currently on your phone.