Do you use one? The blurb on the Blick site says "A curved blade of quality steel removes paint from canvas without damaging it. Decorative wooden handles provide just the right amount of control." Is it worth the $20? Can it be used to remove decades old paint?
I learned of the several years ago when I wanted to remove freshly dried paint, but did what I needed to without one, so never pursued it. Remembered it recently after working on a canvas with old dried paint.
It would need to have rounded corners on it or you would most certainly damage the canvas. I save old credit cards for the purpose of spreading wet paint on canvas because of the straight edge with rounded corners. Works a treat for putting gesso on canvas.
I don't remove paint, I paint over. I sand out the high spots with 80 grit sandpaper. Occasionally break out a random orbit palm sander. and - you probably know that if you're sanding or removing paint from stretched canvas you have to place something under the canvas (like a book) for support.