So this is a BC as Public API removal, this is a compile fail for anyone using this?

"CustomFields" were supposed to be available only for SignUp methods. Because of how the request classes were inherited, this method ended wrongly in the VoidRequest class. No, it wouldn't be a breaking change as there's not a single API method signature that declares a return type of VoidRequest. They all either return an AuthRequest (never a TokenRequest), SignUpRequest (never a CreateUserRequest) or a simple Request or Request<T>.

If people were casting that returned type to VoidRequest (in the case that that method call was returning a VoidRequest class or subclass) and using that method, then yes it would be a breaking change. But that's why we return interfaces rather than classes, to obscure our internals. So I wouldn't consider this a breaking change anyway.

So previously the sign up did work. However it would simply return a void response on success.
So a developer may be checking for null to detect success? Which would now be a BC as the response is not null?

Void class is used in java generics as you can't return "void" nor a primitive value, they must actually return an object. When a method says it returns "Void" it must return the "null value" as seen here.

If they wanted to check success they would just try/catch the request.execute() call. But holding the Void instance is nonsense as they can't do anything with that. So probably, everyone calling this method before these changes would have something like this:

AuthAPI authAPI = new AuthAPI("domain.auth0.com", "clientId", "myClientSecret");
SignUpRequest signUpRequest = authAPI.signUp("[email protected]", "pwd123", "connect1on");
try {
    signUpRequest.execute();
} catch (Auth0Exception e) {
    //Check errors
    e.printStackTrace();
}

and after the change they CAN add the return type (the snippet above will continue working just fine as well👌 ).

AuthAPI authAPI = new AuthAPI("domain.auth0.com", "clientId", "myClientSecret");
SignUpRequest signUpRequest = authAPI.signUp("[email protected]", "pwd123", "connect1on");
try {
    UserInfo info = signUpRequest.execute();
    //use "info"
} catch (Auth0Exception e) {
    //Check errors
    e.printStackTrace();
}

Yes, previous method did work but the response wasn't the right one. Is there an alternative? The only breaking change I can consider is if they were using the nonsense version of the snippet:

AuthAPI authAPI = new AuthAPI("domain.auth0.com", "clientId", "myClientSecret");
SignUpRequest signUpRequest = authAPI.signUp("[email protected]", "pwd123", "connect1on");
try {
    Void result = signUpRequest.execute(); // --> Compile error as Void is now UserInfo
} catch (Auth0Exception e) {
    //Check errors
    e.printStackTrace();
}

They'd need to fix the type of the returned object.

Can't this be stand alone and the old Signup is simply deprecated?

I don't follow. The idea is that every API method returns an interface that just exposes the methods that the endpoint allows. A SignUpRequest would expose a custom fields setter method, for example.