deleted message

You really don't. Once deleted, messages are erased.

If you have a device backup that contains the message, and you do NOT use messages in iCloud, you can restore your phone to the backup albeit at the expense of anything new since the backup was made.

1Snikkers Said:

"deleted message: how do i retrieve a deleted message from my iPhone 8 plus?"

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Check a App on Another Device:

If an app (i.e. Messages) was enabled on any other iPhone or Apple computer, then check the Messages app on your Mac to see if it exists in it. You'd start up your Mac, not connecting to the Internet, and open the Massages app. Then proceed from there with retrieving it. Important: Be certain you do not connect to the Internet while booting up, as syncing would delete it. So, power off your router to avoid the connection.