in my opinion it should be possible to subscribe a hidden page. Many developers don't want the user to navigate with tha page list, but have page links and bookmarks to guide the user to hidden pages, for example with data from another perspective. These pages should be possible to subscribe too. This was a feature once, but it is not possible anymore. Maybe it should be possible to choose wether hidden pages should be subscribeble.
I have found users with similar needs to you Microsoft Idea (powerbi.com) , you can vote the idea and comment to improve this feature. It is a place for customers provide feedback about Microsoft Office products . What’s more, if a feedback is high voted there by other customers, it will be promising that Microsoft Product Team will take it into consideration when designing the next version in the future.