Not quite but close. Titanic, like a lot of movies of the 90s and 2000s, was shot in Super 35. In Super 35, the image is ~4:3, but it requires optical printing or scanning to produce a release print, since the image is both not the correct aspect ratio and also occupies the area used for optical sound.

So it was not "cropped in the theater"—the theater got a standard anamorphic print. To go from the Super 35 negative to the anamorphic print, they both cropped and optically squeezed the image (in the case of the non-vfx shots), and scanned, cropped, squeezed digitally and printed back to film (in the case of the vfx shots). This was a few years before they did full "digital intermediates."