When reading a report in Power BI service, hidden pages are not visible as page tabs, but they still appear in the list of pages when you right click on the navigation arrows:
Clearly, this is a defect -- the pages should not appear in the list if they don't appear in the page tabs in reading view. Users complain about it, because it makes it more difficult to have work-in-progress pages. Hidden pages are not meant to be accessible.
Ideally, hidden pages should also be differentiated when editing the report. I've already posted an idea: Tint hidden pages in list of pages. Hiding hidden pages from list of pages is not a feature request though -- it's a bug, hence I'm posting the issue here.