It would remove a lot of leverage of the twitter mobs if they know ahead of time the target is protected.
[0] To prove prima facie defamation, a plaintiff must show four things: 1) a false statement purporting to be fact; 2) publication or communication of that statement to a third person; 3) fault amounting to at least negligence; and 4) damages, or some harm caused to the person or entity who is the subject of the statement.
Now what you need to do is show damages. Lost your job? Deplatformed or demonetized? Vendors, partners, or clients cut ties? Victim of harassment (online or otherwise)?