In Lincoln Property Co. v. Roche, the U.S. Supreme Court clarified that in an action properly removable by defendant to a federal court based on diversity between existing plaintiffs and defendants, the removing defendant does not have to prove non-existence of some non-party entity, which is a real party in interest and might defeat diversity.
This opinion should be a good source for first-year law students struggling through their Civil Procedure course because it uses plain clear language to explain the removal jurisdiction rules. Where was Justice Ginsburg when I took my first-year Civ. Proc. final?
As for the rest of us, the opinion seems to uphold the status quo. As the Court briefly notes, the 5th and the 9th Circuits already hold that citizenship of a non-party cannot be used by the plaintiff to defeat diversity jurisdiction. That rule makes sense because if a missing party is truly a real party in interest and would defeat diversity, nothing stops the plaintiff, the master of the complaint, from trying to join them as one of the defendants.
Here is Goldstein & Howe's description of the case.