I put the slipper in a zipped pillow case to catch the lint and washed it FIVE times in hot water, with a little bit of detergent and the wash cycle set on heavy-duty. I checked it after each cycle.
Here's about half of the lint. It's better to catch it somehow than to let it clog the washer's drain pipe.
It shrank and felted but as you can see, not nearly enough to make a size 8; or even a size 13! I'm willing to explore and experiment if I learn something along the way; in this project I got better at the Kitchener stitch used to seam the bottom. And I learned if I'm going to veer from an established pattern, I can expect mixed results. More swatch testing was probably in order for this project. So I guess it's back to the drawing board.