Closing apps is not necessary on iOS and hasn't been for years.
When you exit an app, the app gets a few seconds to finish what it needs to do - save a game in progress, stop a timer etc. - and then the system suspends the app. It takes up no memory and processing power by sitting in the background.
Some apps do run in the background, like Mail, Messages, and Music, because you want to be able to receive emails and messages without the app being in the foreground. Other apps can do this, too, but the main point stands - it is not necessary to 'kill' an application.
By killing an app you actually use more power because when you relaunch it the app has to do its initial setup again. This overhead happens every time you launch the app from cold. Launching it from the background uses far less energy.
So, killing an app actually uses more energy.
There is no reason to close all apps in the background.
As a final point, you're in the wrong place. These are the Developer Forums, where developers of third-party apps for Apple's platforms ask each other for hints and tips on coding.
These forums are not where Apple's actual developers chat about new features.
If you have a suggestion, you should raise it at: https://feedbackassistant.apple.com/ but take note of what I've said above - your suggestion is without merit, sorry.