That water's _starchier_ but it's not a myth. Here's Kenji on it: https://www.seriouseats.com/how-to-cook-pasta-salt-water-boi...

(I get what you're saying, spiritually, your pasta water from your giant pot of one box of pasta isn't gonna do much to thicken your sauce. But it's not a myth, just a matter of degree)

If you cook it in a lot less water, and add a quarter teaspoon of corn starch, you can get the same effect. Play with the ratios to taste.

I prefer to use semolina, since that's the same flour the pasta is made of. I find corn starch can add an "off" flavor.

corn starch is widely used because it has no taste raw; a flour based roux needs to be "browned" in oil to eliminate the floury taste (i've tasted the grain of wheat from a plant in a field: tastes floury)

... if you use less water than the amount prescribed on the box it'll be proportionally starchier. It isn't a myth, you can literally see the starch in the water ...

You can see and feel that it's starchy! It has a starchy texture and is cloudy.

Can depend on the pasta too, and how much volume water to pasta you have.