How to view deleted messages in iPhone?

Messages deleted are gone once deleted, if messages are being synchronized using iCloud. Messages deleted when not using iCloud can potentially be accessed by resetting and restoring the iPhone using a pre-deletion backup.

The messages are no longer on the iPhone.

They are stored on your iPhone backup if they were present when the backup was created.

Restore your backup to retrieve the messages that were there at the creation time.

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