The reasons stated by Apple was that our previous app was inappropriately using the VPN API to block content in third-party apps (which is true).
Looking at the overall pattern, Apple let our and many other apps be in the app store for several years. Then, all at once, Apple removed them for a variety of reasons (inappropriate use of various APIs, such as GPS). So, there seems to be a clear correlation with the release of Screen Time in iOS 12. But who knows, maybe they just became more strict about App Store policies and it’s a coincidence that it happened at the same time as iOS 12.
Our new app does not use the VPN API, so it cannot block content in apps. It can only block content in Safari, and sometimes in other “embedded browsers” in apps which use a particular Webkit api (I can’t remember the name).