So holds the D.C. Circuit right now, in an opinion (Carroll v. Trump) by Decide José Cabranes, joined by Judges Denny Chin and Maria Araújo Kahn. The protection, the courtroom held, is not one of many uncommon subject-matter-jurisdiction-based defenses that may be raised at any stage of the case, and thus aren’t waivable. And, the courtroom continued, Trump’s failure to lift the protection early within the litigation (as an affirmative protection in his Reply to Carroll’s Grievance) did certainly waive the protection.
None of this resolves whether or not the protection, if raised promptly, would have certainly precluded the civil lawsuit towards Trump, nor does it communicate to the deserves of the lawsuit.