There are APIs which are marked as public and are currently being used in Production by some applications. The plan is to change some public APIs to private but it appears this cannot be done unless all mapped applications are removed. If those mapped applications need to be removed, it will cause Production downtime.
Can live public APIs be marked private without downtime?
API PORTAL 4.2.X
Unfortunately, there is no way around taking an outage. The portal restricts this action for all versions of an API to prevent unwanted sharing. This is done on purpose to make sure that an application has not been assigned to an API by accident, since all existing apps will still have access to that API.