John Gruber has a lot of influence in the Apple community, and rightfully so. He is right most of the time. And when wrong, he is smartly wrong, never stupidly wrong. But in the case of one of his recent articles, he couldn't be more wrong:
He has overstated his case. And in doing so, as generated bad advice for many users. As you can see in the above video, closing unnecessary apps has an immediate and demonstrable effect on at least one accessibility feature. The fact that I can show you one example means that there are likely many others that I do not know about, or that are not so easy to demonstrate.
Toward the end of his post, Gruber acknowledges that some apps are bad actors that can have a negative effect on the system. This should have given him pause when recommending that background apps should not be closed. The fact that there are known apps that cause problems means that apps can cause problems. The truth is that we don't know all of the problematic apps.
I have been running into this problem for a rather long time. No one app seems to be the culprit. If I can't sort it out knowing what I do about the system, the average person does not stand much of a chance. Since you have no idea which of your apps might misbehave, it makes more since just to close some of them periodically.
Gruber makes it sound like there are only one or two bad actors in a sea of perfectly behaving apps. That does not seem to be the case at all. So if you are having some mysterious performance issues with your device and notice you have lots of apps open in the background, you might try closing a few.
John Gruber's advice is well-sourced. But it happens to be wrong. Refer to the video above if you have any doubts.