Это значит, что автор думает, что это незаконно, но пока не было ни одного соответствующего судебного решения, и поэтому нельзя быть на 100% уверен.

It means that no court has yet decided whether something is legal or illegal. In the American justice system, some laws are created by the Legislature. But those laws are not always clear about how they apply in a particular situation. For example, does a particular law about privacy mean that an employer cannot ask for certain information? So courts have to decide what the law means in a particular situation. One party (maybe, the employee) has a theory about what the law means, and files a lawsuit to “test” that theory in court. The court then decides if the employee’s theory is right or wrong.

Thank you for this example, Marina

Well, imagine that you are being accepted to work in a company where your duties would include a lot of online chats, Internet browsing, etc. All for business purposes, of course. You are about to sign the contract when you see something along the lines “...The Company reserves its right to supervise (check, regulate) your usage of Internet resources through The Company’s domain...” You ask: “Excuse me, what does it involve?” The answer would be: “Oh, don’t worry about it. Just make sure to let our security team know your password when you finish setting up access. They check all of us twice a year.” What would you do, sign it, or forget about the job? “Untested in court” means that up to date of the article there haven’t been a person whose complaint had survived long enough to get to a court hearing.