It also describes how to hide the Messages app from Siri Spotlight search and includes other workarounds.
Instructions in this article apply to iPhones with iOS 14 through iOS 11.
When you delete some kinds of items from the iPhone, they aren’t removed.

Lifewire / Luyi Wang
Lifewire / Luyi Wang
When yousync with iTunesor iCloud, the items you marked for deletion are erased.
When you delete a text and then sync your iPhone, the message is gone for good.
Deleted messages don’t appear in a Spotlight search if Spotlight isn’t looking for them.

You control the apps Spotlight searches and which it ignores.
From the iPhone home screen, tapSettings.
In older versions of iOS, tapGeneraland then tapSpotlight Search.
Scroll down and tapMessages.
Move theSearch & Siri Suggestionstoggle switch to theOff/white position.
It deletes your music, email, apps, and everything else, too.
The same is true when yourestore the iPhone to factory parameters.
This procedure returns the iPhone to the state it arrived in when it came from the factory.
It deletes everything, but your deleted text messages will definitely be gone.
Snapchat works this way, but it’s not the only option.
That’s because it could be stored on your phone company’s servers.
Normal text messages go from your phone to your phone company to the recipient.
In most cases, that means the phone company retains a copy of messages.
These can be subpoenaed by law enforcement in criminal cases, for example.
Go toSettings>Messages>Keep Messagesand verify it is set toForever.
The other options are 30 Days and 1 Year, after which your messages are deleted automatically.