I’m already sponsoring Voyager on GitHub, but now that the app even exists on the AppStore I really hope you’ll consider adding an actual Paid Subscription, as many of your users (probably most) don’t even have a GitHub account.
It doesn’t really need to unlock anything; maybe just let people opt in to be added to a credits list or something. All you need is an in-app notice that explains the nature of this project along with a request to support the project with a paid subscription. Judging by the amount of people willing to pay for Sync, I imagine there are plenty of happy Voyager users who’d appreciate the opportunity to pay for the excellent Voyager app to ensure its longevity.
Whether the app store’s subscription facilities should be used is unclear, but I’m leaning towards an independent solution that would still work on the PWA and doesn’t get hit with the exorbitant 30% platform fee.
Advertising any kind of payment / subscription that does not go through the store through which the app has been distributed, is prohibited by apple and google (not 100% sure about google, but fairly certain).