Let's say once you install the app and open it for the first time. You are presented with a PayWall, that you cant skip. In order to use the app, you have to Subscribe via a FREE TRIAL Subscription. Once the free trial ends it will automatically charge the user for the Subscription price. Unless they cancel it before, making the app useless forever until they decide to pay the Subscription.
Does Apple allow that ?
Apple's website about business models states the following about the freemium model:
[It offers] the option to pay to enhance or customize the experience.
Therefore, I don't think that Apple will allow you to implement a freemium app without providing actually free content.
The auto-renewal of the free trial into a paid subscription is the "normal" behavior of Apple's free trial.
Update 2024
Apple has removed the part mentioned above from the documentation. Maybe this is allowed now or always has been, and Apple removed that to prevent confusion.