How can install an app I purchased on the App Store which is no longer on the app store?
Apps on the App Store (iOS, tvOS) and on the Mac App Store cannot be re-downloaded if


the developer removes the app
Apple removes the app for various reasons (e.g. violation of App Store policies, copyright infringements)


Since 2015 Apple removes apps from your purchase history as well. A well-known example is "Monkey Island 2" by Disney. It was removed with no option to re-download a copy of the game.

Try to contact the developer of the app. Maybe he/she can send you a copy of the app.