Oppenheimer is being released next week.
People on social media are realizing that few cinemas can actually project actual IMAX, and that the vast majority of spectators will therefore watch a cropped version of the IMAX scenes from the movie, since IMAX has the unusually tall aspect ratio of 1.43:1. Hence the viral buzzword "Croppenheimer".
I don't think it matters much, since Christopher Nolan has shot IMAX scenes ever since The Dark Knight (and nobody complained), and framing seems to be designed for a wide aspect ratio anyway even for IMAX sections.
Even though I've never seen a movie in IMAX, my understanding of the concept is that it's supposed to immerse the spectator in the movie by occupying as much vertical field-of-view as possible, in addition to the horizontal field-of-view that is already plenty occupied by modern movies and their wide aspect ratio. Composition is made to include key elements of the scene appear at the center of the frame, such that it still works on wide "cropped" aspect ratios, but the image becomes taller, including non-key elements top and bottom for the sake of immersion in the IMAX scenes.
Contrarily to what the "croppenheimer" buzz suggests, IMAX should be thought as a vertical extension of an already wide image, rather than a wide image being a cropping of an IMAX square-ish image. The official IMAX site says that IMAX images "expand vertically to fill the entire [IMAX] screen".
Anyway, I was originally lost on all those notions, and this was the occasion to read about the subject. The best source I found about what is going on with Oppenheimer, format-wise, is the technical specifications from this page from the official website. It's undecipherable without some notions about film format, which I gathered from Wikipedia. I eventually decided to make the following infographic to summarize the situation. Share it as you wish.