Since Apple doesn't offer trials I have no issue with a dev offering some limited free download with IAP to unlock. The limits need to match the app in some way, like a limit on total queries or a timer.

Subscriptions have to be either something that has recurring costs to the developer (like they depend on a 3rd party API that charges for access) or the dev is upfront that it's an optional opportunity to support them. I'm also fine with some nice-to-have features locked behind a paywall if they are not core to the app functionality.