In iOS9 there won't be a Newsstand app anymore. What will happen with apps that are using Newsstand? Is there some solution, or will developers just have to re-implement apps to no-Newsstand and count on users to update their apps before they update to the new iOS version?
Yes, That's true. According to iOS 9 updates here.
Updates in iOS 9
Changes are coming to Newsstand this fall with iOS 9. While publishers can still distribute periodicals on the App Store with auto-renewing subscriptions, the Newsstand app will no longer be pre-installed.
What to Expect
Individual Newsstand apps will now install directly to the Home screen. If a customer had Newsstand apps installed prior to updating to iOS 9, Newsstand will become a folder titled “Newsstand” and their existing apps will be placed in this folder.
Newsstand app icons will no longer be the cover of the latest issue. You should review and update your App icons.
You will still be able to update your issue preview screenshots and metadata without submitting a new binary.
Background downloads will continue and existing subscriptions will be unaffected.
You can also refer to News App - Developer Library for more information