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Hey there Robb0123,

We wanted to provide an article for your reference that discusses the new feature of recovering a deleted text message on your iPhone, but it does require iOS 16 or later, and your iPhone 6s Plus can only go up to iOS 15.7.5: Recover deleted text messages on your iPhone or iPad

While the method above wouldn't be possible as you have an older iPhone model, if you do not have iCloud Messages already enabled, then a backup made prior to deleting the messages could be used to restore your iPhone. If you do have iCloud Messages enabled, there's not a way to retrieve those deleted messages.

Should you decide to restore from a backup, please be aware that it does erase what's currently on your iPhone, and replaces it with what was stored in the backup when it was made. Should you decide to do this, you can restore from a backup using these steps: Restore your iPhone, iPad, or iPod touch from a backup

Cheers!