polygonum wrote

How dangerous would it be to use a phone or tablet in the
bath whilst it is connected to a mains-powered USB charger?

Not dangerous at all if the charger is designed properly so that no
matter what fails you can ever get any mains voltage on the USB.

Quite dangerous with the worst of the chinese **** that can do that.

Obviously it could be dangerous to the device itself if you drop it into
the water!

Not with a properly designed charger than has the USB
electrically isolated from the mains even if the device fails.

Obviously a 5 volt low current supply isn't usually life threatening.

Never is in fact.

Obviously we have rules about sockets in bathrooms. But a long USB
cable to a socket in another room seems a likely way round that.

But if something is wrong somewhere, such as a mis-wired
USB charger, could there be any appreciable danger?

Not miswired so much as if it isnt properly designed
and does fail when you are using it in the bath.

Is there in future likely to be a bathroom-safe USB
socket next to the shaver and toothbrush socket?

Unlikely given that there isnt any real need to charge anything
in the bathroom except shavers and toothbrushes.