Deleted Messages

Do you have iCloud messages enabled?

Yes, or no.

If yes, stop. The messages are gone. If no, proceed.

Don’t know if iCloud messages are enabled? See if the checkbox is enabled: Use Messages in iCloud - Apple Support

If that checkbox is enabled, the messages are gone.

Do you have complete backups of your iPhone?

Yes, or no.

If no, stop. The messages are gone. If yes, proceed.

Do you have a backup of your iPhone created prior to when he message was deleted?

Yes, or no.

If no, stop. The messages are gone. If yes, proceed.

Are you willing to delete all changes on your iPhone and anything that’s been added, deleted, customized, or changed since the backup including all app data?

Yes, or no.

If no, stop. If yes, and you are willing to revert everything in your iPhone, restore the backup.

To restore a backup: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you're using iCloud messages to synchronize your messages, then the deleted messages are gone.

If you were not using iCloud messages and if you have backups from prior to the deletion and if you are willing to revert everything else on your iPhone to what is contained in the backups that are possibly containing the deleted messages, then you might get them back.

Otherwise, deleted messages are deleted messages: deleted.