Interesting - I recently got myself an S9 and my wife a OnePlus 6T, and the OnePlus gets much better battery life (with fewer apps and overall lighter usage) - but now I'm not so impressed if that's how it's achieved.
It's a fairly divisive issue - I can understand developers getting frustrated if their app's functionality is broken, but I do think this should be more in the user's control. A permissions-based model for backgrounding would be better, but there's too much permission prompting as well on Android (and this is something you can't back up - so on any restore, you're left 'allowing' permissions in every app you use in a new way - yes, when I select the camera mode in an app, I want to allow use of the camera - gets tedious across a bunch of different apps).
My S9 (or maybe it's just Android) also puts a notification up if an app continues to run when I click 'back' out of it (or some other heuristic - it's not for every app, but it does a good job of identifying which are running) - which I think is a better compromise. It makes it much more visible which apps are consuming resources, and tapping the notification takes me to the App Info page with the 'Force Stop' button at the top. It's a bit noisy (it appears even for the stock music player, where obviously I want that to continue running), but it serves well as forcing visibility of battery hogs (whether it's through incompetence or nefarious background data collection), and reassuring to see it there for the apps I want running in the background. I would rather see more of this sort of visibility - maybe as well as just overall battery usage, the battery use in settings should show battery usage by apps when the screen is off or the app is not in the foreground. As the device owner, I want to be in charge of which apps get that privilege.