In issue #3556730: Replace jQuery code and remove jquery_ui dependency the basic_cart.js was refactored and all jQuery code was replaced with plain JS code. This made the dependency on the jquery_ui contrib module unnecessary.
jquery_ui was removed from basic_cart.info.yml dependencies. To prevent the jquery_ui module from being removed from the file system, leaving administrators with a missing module, it was added to the new composer.json file as a dependency.
Furthermore an update hook was implemented to simply print a suggestion message to manually uninstall the jquery_ui module if no other modules, themes, or custom code depend on it.
jquery_ui will later be removed from the composer.json on the next major release.