I saw screenshots of the open matte Russian iTunes Infinity War on another forum & it looked cropped on the left hand side (but expanded top & bottom).Technically that iTunes format would be "open matte" not IMAX
Think it might be based on the version they use for airplane in-flight entertainment as that was similarly cropped on the left (& expanded top & bottom), when I watched it on a flight.
I’d guess the Disney+ IMAX version won’t have the same cropping.