Here’s how you can prevent in-app purchases on an iPhone in 2019. This guide is for iPhones with the current iOS version 12.1, as of March 7th 2019.
Go to Settings / Screen Time / Content & Privacy Restrictions.
Tap the Content & Privacy Restrictions button to enable it.
Then, on the same screen, continue to iTunes & App Store Purchases / In-App Purchases.
Select the Don’t allow option.
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