An Old Purchase
What can be done if a purchased app does not allow you to reinstall without paying again after 3 years? The app is now a paid monthly subscription.
iPhone XR
Then you would have to abide by the developer's new payment scheme. The new version of the app would be considered a version upgrade, not an update. Updates are free, upgrades can be paid.
ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2.8; iPad Air 2, ipadOS 13.6.1; iPhone 8+, iOS 13.6.1; TV 4th Gen, tvOS 13.4.8; HomePods, audioOS 13.4.8 and Airport Express w/ Airplay 2