Unreal Engine isn’t a shared iOS component. Apple doesn’t care about signatures on individual lib/so files: only the signature on the published app package. When you make an app or a game using an existing engine then only you become responsible for it.
A ragtag band of devs all banned-for-life from Apple’s stores could put together a new app-engine or game-engine that other devs use as the basis for their app and provided that those apps don’t break the rules about undocumented APIs or malware (and no-one in Apple’s higher-ups gets suspicious...) then those apps will appear. I don’t believe Apple has any way to automatically ban or delete all apps that use the same third-party libraries by using their macOS Notary system or iOS Certificate revocation system - they’d have to inspect every published app-package individually (granted, this would be automated, but you know what I mean).
A ragtag band of devs all banned-for-life from Apple’s stores could put together a new app-engine or game-engine that other devs use as the basis for their app and provided that those apps don’t break the rules about undocumented APIs or malware (and no-one in Apple’s higher-ups gets suspicious...) then those apps will appear. I don’t believe Apple has any way to automatically ban or delete all apps that use the same third-party libraries by using their macOS Notary system or iOS Certificate revocation system - they’d have to inspect every published app-package individually (granted, this would be automated, but you know what I mean).