A modified 3.1.1 rule in the guidelines says that apps are now able to offer a free time-based trial period using no-cost in-app purchase options that will temporarily unlock app functionality.
So, for example, you will be able to make a $0.00 in-app purchase in an app to unlock a free trial period that lasts for a set period of time, usually a few days to a week or two.
With this change, all apps in the App Store will be able to free trials to customers should the developers choose to implement the feature.
Prior to the guideline update, free trial periods for App Store apps were available only for subscription-based apps, where you had to agree to make a monthly or weekly payment to get access to a free trial.
A lack of free trials for apps has been a major complaint for both the iOS and Mac App Stores, and especially so on the Mac App Store, where apps are often more expensive and offer more extensive functionality that customers may want to test before making a high-dollar purchase.
Apple implemented several other changes to its guidelines, including restrictions on subscription apps that attempt to scam people out of money and new rules for remote mirroring apps like Steam Link.
Article Link: Free Trials for All Paid Apps Now Possible Thanks to Updated App Store Guidelines