WordPress creating images if uploaded image is greater than 960px on one side?

WordPress v4.4 introduced an additional image size named medium_large which is is 768px wide by default, with no height limit. There is no UI for the medium_large but the size can be changed by updating the options:

/**
 * Updates the medium_large image size. 
 * It's only necessary to execute this code once.
 * Ideally you'd want some kind of UI to allow the dimensions to be specified by the user.
 */
function wpse238808_update_image_size_medium_large() {
    update_option( 'medium_large_size_w', 0 );
    update_option( 'medium_large_size_h', 0 );
}
add_action( 'init', 'wpse238808_update_image_size_medium_large' );

Changing medium_large‘s width and height to 0 like you’ve done with the other image sizes will prevent the additional images from being generated.