How on earth would that be a feature? If you have to show a popup on launch for something like that then it's a user experience failure, no matter if it's your fault or Apple's. Either way, it means the user has to treat your app differently to every other app on their phone. That is bad.
"It's user error" is a weak excuse if any kind of significant number of users are doing it.
An app that has been force quit can still receive remote notifications just fine. I don't think the average users knowledge is so nuanced that they understand the distinction between a remotely-sent notification that always appears and a locally generated one that's the result of a background process that doesn't.
I think most users expect to receive notifications from apps (especially communication apps) even if the app doesn't appear in the App Switcher carousel. What people expect from their phones and computers are wildly different here.
The feature in my mind was an easy way to temp silence an app without going into settings.
I still think of it as user error if they force quit an app but still expect it to run in the background. Why force quit it?
I could see how it could be a UX failure for the OS but I don't think we can blame the dev for that. Apple should clarify what force quit does. I think the advent of webapps that don't really exist on your phone at all also causes confusion: the app has been updated with more content each time you use it, even if it's not running in the background, but to the user that's indistinguishable from the app running in the background.
"It's user error" is a weak excuse if any kind of significant number of users are doing it.