They do not want you to swipe-close all apps, and used to very publicly ask people to stop doing that every year during iOS keynotes. They want you to trust iOS

They do not want you to swipe-close all apps, and used to very publicly ask people to stop doing that every year during iOS keynotes. They want you to trust iOS to swap apps in and out as needed for performance, and all you're doing by forcing the apps to close is ensuring slower startup next time you use them, since they'll be launched from scratch rather than restored from cache. So this is not a "missing feature," it is you using the system in a way they don't support. They won't likely add the feature you want, but you could consider leave the apps alone, and only swipe-closing them if you want privacy.