Apple now allows unlisted apps on the App Store
Making an app unlisted means that it won't show up "in any App Store categories, recommendations, charts, search results, or other listings," according to Apple. The only way to get to the app is with that link—or through Apple Business Manager or Apple School Manager.
Unlisted apps are ideal for “limited audiences,” such as guests at a special event, members of an organization, research study participants, or a specific group of employees. To make an app unlisted and obtain a link, developers will first need to submit a request to Apple.
For more details and for links to the forms and steps in the process, visit Apple's developer documentation on the topic.
