Yeah, getting low-carb food on airplanes is struggle. As others have said, bread and pasta are cheap. I try to always have some beef jerky, nuts, etc. with me in case the options don't fit with my diet. When traveling coach and/or back in Covid with no food service I'd take lettuce wraps with bacon, cheese, avocado, etc. They wrap up nicely and take up minimal space. A couple boiled eggs in a zip-lock are also good.