Only 38 of the 65,000 developers applied to use the payment system outside the App Store ecosystem according to testimony given by Apple representatives during the trial of Epic Games' lawsuit against the company. This equates to only 0.06% of all developers offering their apps on Apple devices.
This according to Epic Games is not because payments outside the ecosystem are not good. The opposite is because Apple still charges a 27% cut-off fee to developers who want to offer payments outside of the ecosystem. After banking costs are factored in, the cost to developers is more than the 30% cut Apple currently takes on the App Store.
Apple was ordered by the court to allow payments outside the ecosystem in 2021 while the trial of the Epic Games lawsuit was still successful. Epic Games said that Apple's efforts since the directive was not satisfactory. The case is still ongoing.