Probably a bit of gossip, rumor, trivia and/or hype, though I seem to recall something to that effect way back when.
Trivia from the Amazon VHS listing:
Michelangelo Antonioni's original ending was a shot of an airplane sky-writing the phrase "Fuck You, America," which was cut by MGM president Louis F. Polk along with numerous other scenes. Louis F. Polk was eventually replaced by James T. Aubrey, who had most of the cut footage restored, but without this final shot.
From a Senses of Cinema interview with Rolando Caputo of the Cinema Studies Program at Melbourne's LaTrobe University:
Saul Symonds: The first thing I wanted to ask you wasn't actually a question, it was more a point I wanted to clarify. I was reading about Zabriskie Point (1970) and this website said that the final scene was originally of a plane skywriting the words, "Fuck you America," and of course it said that the studio made him delete that. I was curious if you're able to verify this.
Rolando Caputo: I'm not sure about the skywriting, but I have heard people say that there was some variation regarding the ending. I believe that the issue was about the female character getting back into the car after the explosion. There was talk that Antonioni wanted to end with the explosion and leave it somewhat ambiguous whether it was a projection of her imagination or whether it had a real status to it. The shot of her getting back into the car makes it appear that it could only have been a fantasy projection of hers. The ambiguity has been removed. But this is pure speculation, I've seen no definitive evidence about alternative endings to the film.