Disabling background scrolling when a mobile menu is active on iOS can be achieved using the touch-action and -ms-touch-action CSS properties, which help control how touch input is handled by the browser. However, keep in mind that -ms-touch-action is a proprietary property and is specifically for Internet Explorer, not iOS.
/* Disable touch events for the element with the menu */
touch-action: none;
/* For Internet Explorer */
-ms-touch-action: none;