If the OS of an ECS Instance in an AS Group Is Upgraded, Can AS Automatically Add Instances with the Newest OS During the Next Scale-out Action?

No. You need to compile a monitoring plug-in and install it on the target instance. Whenever the OS of this instance is updated, the plug-in will call the relevant APIs automatically to create an image from this instance, create an AS configuration using the image, and then replace the AS configuration used by the AS group with the newly created one.