It's not hidden at all, Claude pushes it even tho it poisons the context after every edit with false positives because it's always out of date. This feature should be hidden given how half baked it is.
Networking as a whole can easily be controlled by the OS or any intermediate layer. For controlling access to specific sites you need to either filter it at the DNS level, which can be trivially bypassed, or bake something into the application binary itself. But if you are enabling untrusted code and giving that code access to a TCP channel then it is effectively impossible to restrict what it can or cannot access.
(haven't tried it myself)