Why that plugin should be added?
Most of app-stores (Android, iOS) requires the usage of Google Play Billing and In-App Purchase.

What features this plugin should have and how it should work?
Make possible to make payment, receive payment notifications and check payment status.

What alternatives you've considered?
External payment/gateway is not allowed for Google Play/App Store.

Is this plugin implementable on all OSes (Android, iOS, macOS, WASM, Windows...)?
iOS/macOS: https://developer.apple.com/documentation/storekit/in-app_purchase?language=objc
Android: https://mvnrepository.com/artifact/com.android.billingclient/billing and https://developer.android.com/google/play/billing
Javascript: https://developer.mozilla.org/en-US/docs/Web/API/Payment_Request_API (nobody cares)

Did you known any particular limitation for specific OS?
Each API is different.