In addition to changes allowing for apps like Valve’s Steam Link noted earlier this week in Apple’s revised App Store Review Guidelines, it appears that Apple has now also opened up the gates for time-based trial periods. MacRumors discovered the change in section 3.1.1 of the App Store Review Guidelines, which adds a section stating that non-subscription apps may now offer an explicit free time-based trial period as a zero-dollar in-app purchase.
Specifically, the new section 3.1.1 notes:

Non-subscription apps may offer a free time-based trial period before presenting a full unlock option by setting up a Non-Consumable IAP item at Price Tier 0 that follows the naming convention: “14-day Trial.” Prior to the start of the trial, your app must clearly identify its duration, the content or services that will no longer be accessible when the trial ends, and any downstream charges the user would need to pay for full functionality.
While subscription-based apps have long offered a free trial period before the subscription begins, this change will be of benefit to apps that don’t otherwise offer subscriptions — and may indirectly benefit users by removing an incentive for developers to go to a subscription model merely to gain support for a free trial.
While some apps have worked around the free trial issue in the past simply by offering functionality directly within the free version of the app, this move should make it easier for developers to set up and track free trials by using the App Store’s built-in IAP system, rather than having to code their own limits.
The ability to release free trials of apps has long been requested by App Store developers, and in fact was the leading request from a group of developers that formed a Developers Union earlier this year.