Actually - GD - Here's the British government telling teens this...
Read here. It's up to you whether it's scientific enough.
While I do think that a minimum is kind of arbitrary, it's there for those that tend to ignore these 'rules of thumb' - particularly children and teenagers, who exert themselves without paying much attention to rehydration, and adults who suck down Coca-Colas and coffee all day long. I think that most feel comforted with some sort of number to go by. Probably a human nature kind of thing.
At any rate, there's little sense in 'debunking' it as some god-awful myth... Though it's been put on a pedestal of sorts, it's better to think of it as the aforementioned rule of thumb.
With the thirst thing, I'm not going to bother doing anything more than typing it into google and putting up the most official looking link with the terms "by the time you're thirsty, it's too late."
But, it stands to reason that since thirst is a signal of dehydration... not anything incurable or so awful... you could have prevented such thirst by drinking plenty of water to begin with.