I personally know a very rich family who I know for a fact has sued Forbes when they were once put on the list.

I know this because I was doing work IT work for the lawfirm that did the suing. As it turns out, in the US lots of this depends on if the person is deemed as a "Public Person" or not.....if a person is classified as "Public" then there is no real privacy awarded to them, whereas if they are "Private" then there are much stricter laws against defamation, etc.

To maintain your status as "Private" you basically need to never issue any public utterances. That means, no interviews, no speeches, no social media, etc. So, it wouldn't surprise me if there were lots and lots of megawealthy who are totally unknown to the general public.