I thought: surely there must be a way to unmount the USB storage. It's counter-intuitive to just unplug a device without first unmounting it in any OS. I looked (and looked some more) in System Preferences and in Files but still thought: I'm missing something here... Nope: Apple's guidance is just to unplug the USB device.
As of this writing (20191211) you just unplug it. I'd qualify that advice with the further suggestion to check the device's light to see it's not doing anything before ripping it out.
Figured there would be countless others thinking "Surely there must be a way to properly remove USB devices from my iPad...". Short answer: nope.