Android allows that in settings. But AFAIK it's just a software setting, the data lines are still physically connected.

The reason why there's no physical hard disconnect available in settings comes back to what I said earlier, cost and convenience from the manufactures' perspective.

They don't want to spend 2 cents on some mosfets for HW disconnects, and they don't want to nag the user every time they plug in the phone to charge as that would hurt the UX. They want the device to sync up automagically to whatever device you're pugging it into. That's what USB was designed for, convenience.

Plus, if you nag the user every single time they plug in the USB with a security prompt, they will just get desensitized and instinctively click 'YES' all the time.

You also need to have the USB connected on boot, for update/flash purpose.

That is what my android phone does, I have to toggle it to allow data every time.

Funny you mention that when literally all flagship phones (both iOS and Android) have a "Do you trust this computer?" popup that does exactly what you described. By default, phones don't do any data transfer over USB.

That only works if other endpoint tries to act as usb host. While if it mimics usb device (classic BadUSB), then AFAIK any phone would accept without any questions.

Classic BadUSB acts as a keyboard and opens a terminal on your PC. What exactly can a bad usb device do to a phone?

This whole thread is bizarre. A whole lot of fretting over an attack that nobody bothered to describe.

It can act like keyboard an mouse that are valid input devices on phones too. With control of user input badusb device can download and install malware on the phone.

Now we're just taking in circles. What exactly can a malicious USB device do to a phone? On Windows it can press Win+R, but there's no equivalent on mobile.

Which keystrokes are you talking about and on which OS does that attack work? I can't think of anything that would work on my iPhone.

Open chrome (swipe up with virtual mouse, type chrome, click first entry), focus on address bar, input url of malicious apk, download it, open, confirm installation from unknown source.