Infinity, a lesser known open-source third-party Reddit app for Android, has announced that it will be switching to a subscription-only model. This would mean that the F-Droid version of the app will be delisted and the API key currently in the source code of the app will be removed. As of this post, the developer is currently not interested in converting Infinity into a Lemmy client, but encourages others to look into doing it themselves because of the app's open source nature.
How is a FOSS project gonna enforce this? By providing only binaries on GPlay with the dev's API key, and then a free version for entering your own API?
This is a common approach if API access is paid.
Not doing this is equivalent to lighting your money on fire because someone WILL extract your API key and start scraping.
Reddit won't allow the dev to create an app where users can enter their own keys. So it will still be FOSS but you'd have to build it yourself with your own keys, if you can somehow obtain them. Thus there's no point in putting the app on F-Droid.