It’s still a choice for develop cross-platform. For those UI stuff which couldn’t flexibly or can’t integrate with game entity, but it needs to be used depends on platform currently building.
I prefer to use game entity from the class it’s developed for as it is, and turn to multi-platform library for integrate the UI stuff (such as dialog, window, etc).
But in any rate, doing thing like in-app purchase, notification, etc needed to tie to the specific platform (anyway that section of code is outside of the game code). I don’t think there’s such a library to develop once and have all the abilities from all platforms.
That’s my opinion.