Recently there have been great pressures mainly from the European Union regarding the release of alternative stores on IOS systems. This means the loss of the App Store’s dependence on the publication and maintenance of the Anki application on Apple systems. My suggestion is that the developers of the application post the application in an alternative store, such as AltStore - allowing more users to access it without the high fees charged by Apple
Your suggestion would have pretty significant impacts on the revenue stream from the app. There are reasons why the app is paid – Why does AnkiMobile cost more than a typical mobile app? - Anki FAQs . Have you considered any of that?
The few percent that you save by distributing it another way outside the App Store might be relevant for big players like Spotify who can really capitalize on that because of their size, but maintaining your own payment system is adding a lot of overhead. You have to identify and contract with a good, cheap and reliable PSP, implement all the payment flows in the app, you need support staff to handle payment issues, refunds etc., so whether that really substantially makes anything cheaper for a small- to medium-sized company is questionable. And lastly, most of your users would still use the App Store anyway (at least at the current point of time).