First things first, here’s something mentioned recently …
To be more precise … make the Developer options visible by tapping Settings - About phone - Build number a few times until you are being declared a developer, then enable Settings - System - Advanced - Developer options - Kill app back button.
It’s not perfect, it seems killed Apps stay in the App list you get with the square button, which looks odd, but they are closed. Try to open them, they start from scratch, they don’t return from the simply inactive state they are in when they are not closed.
Else …
Yet you seem pretty sure you know enough to specify that some feature has to work in a certain way on a smartphone for the whole universe of something or other. Does that go well together?
But you are basically saying Android should have respected what the lord desktop OS told it (Thou shalt keep my button to close an App!).
You want to have change when it suits you, and you want to have no change when it suits you. Fair enough, who doesn’t? But no need to be overly dramatic about it.
I guess many users including me had a little “Oh! So they are not much into closing Apps in these parts.” moment when having their first Android (or smartphone) experience, and you are not alone in wanting a close button, but that doesn’t automatically mean your demands would be met with unanimous agreement, or that you would even be in a majority.
And why would that be? It could just be no big deal for many others, and it could be that OSes (not only Android) could point to technical reasons (which might of course be a made up pretense hiding the respective sinister plot of the season, if you don’t like those technical reasons) … example:
https://www.wired.com/2016/03/closing-apps-save-battery-makes-things-worse/
If you assume an App does malicious things when it is un-closed in the background, how do you assume it does no malicious things when you are using it in the foreground? It’s the same App, the same code running on your phone.
I think if you are using an App you believe you would have to absolutely stop to preserve data “at home”, it’s not the OS having the problem. It’s the App or it’s you.