Your question was does anyone carry a short boot knife? I think you meant as a defensive weapon. I have but will never will again, just no real defensive value in my view, people think they are but just ask any cop if there was ever a cop saved by a boot knife. I do not know of any. I carried knives and guns in boots in law enforcement and off duty. And I carried the original Gerber Mark II fighting knife in the military. The general rule is the blade must be 6 inches or more to be effective as a fighting blade, otherwise you are way too close to the other guy while you are making shallow stabs that will not necessarily shut him down, so there is lots of controversy about the value of a short knife when each of you must stab the other guy many times before one of you will then have a fatal or incapacitating wound. The guy with at least a 6 inch blade has the odds of winning. If both have short 3-4 inch blades then it is a bloody mess that lasts a long time and the only really effective strike would be to the head or jugular or the spine from the back, pretty hard to do when you are both fighting.
That being said, I did carry one often undercover when a gun was out. But if I could have a 22 or 25 I would always choose the gun. A mouse gun in 32 or 380 is pretty effective out to 20-30 feet on head shots and has 5-7 rounds and only weighs 10-12 ounces. Your short knife is only effective at arms length and a 4 inch blade only penetrates 4 inches, a 380 with ball ammo will penetrate 10-12 inches. And the gun is only about 6 inches long, barely into the top of the boot.
The other reason I do not carry one is that I can carry an traditional folder with the same blade length in a pocket or on a clip along the waist so I never have to bend over and pull up the jeans or any of that, my folders open one handed so they are already there near the hand and quick to deploy. And when the cops come, they just look like a big pocket knife not some dagger. It should not matter but it does if you use it.
They do carry well in a boot and you will not even notice it after a few days. That could also be said about the small handguns. FWIW