This is called an optical reduction, and optical reductions didn’t come into place until the 1990s when they were finally used for theatrical re-releases of classics such as The Wizard of Oz, Gone With the Wind and various Disney animated classics to show them pillar-boxed in theaters.
No! Don’t alter the movie with that crap. Disney already did it with “DisneyView” option on Bly-rays which put special artwork in the side panels. Unused areas in the screen should be invisible as in black and not illuminated by a fake curtain in a darkened theater.
What feels like theater to me is an image projected on a screen with curtains on the sides that open or close to the exact shape of the image without black bars, whether it be 1.19:1 or 2.55:1. But it’s lonely in this camp...