In all the years that Apple's App Store has been in existence, apparently, they have never allowed a company to hide one's app from the store. Doing so would make the app available only via a direct link. Why would a developer want to hide an app? Perhaps it's for a one-off or annual event like Comic-Con. Or the app could be limited in use by employees of a singular company. Regardless of the use cases, Apple has finally implemented this obvious feature according to Amanda Yeo at Mashable.
It will also help declutter the App Store, so you don't have to scroll through 50 billion irrelevant corporate apps whenever you do a search.
Mashable
In order to unlist an app, developers must submit a request and wait for approval. The app has to be completed and ready for release though, so it can't be in beta. If approved, the app will be unlisted and a link to the App Store listing will be generated, which developers can then write in lemon juice to slip to their secret club at recess.
Leave it to Apple to introduce this feature and stymie developers by making it a request to the App Store team. They easily could have introduced a toggle within the Developer system. I don't understand why it took so long for a clear-cut feature to come to developers. I really don't understand why it isn't a self-service item. Apple must really love red tape.