I don't think that term was ever meant to mean impermeable: it was (and is) a symbolic gesture of investment banks resolve to attempt to keep the various sides of the house separate. Not possible, of course: but at least they say they are trying, so the story goes. It's more of an attempt to make the diviing line at least visible, to be used as a guide. Ultimately, like trading on non-public information, it's something you're unlikely to see stopped except in egregious situations.
Uncle Joe used to say: Trust, but verify.
/jordan