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I think the "Close All" button may only appear at the far-left of the Recents view when you're using the Pixel Launcher.
Curious, though: why do you need to close all apps? Just it's a list of recent apps, not an indicator of what might actually be running in the background on the phone. Apps listed there may have stopped running already, and others may be in the background without appearing there.
You can swipe up on an app to dismiss it from the Recents view. Why would you need to dismiss all of them at once?
Trying to manage the apps which the phone is already managing without my help doesn't seem very convenient to me.Just convenience I guess.