I live in eastern Washington State, and composting toilets are legal within circumstances whereas most people who would want a composting toilet, could have one.
Most interestingly... if you have a traditional plumbed flush toilet available in your home, then you can have a composting toilet.
Because composting toilets don't actually make compost, the majority of the rules are about where not to dump the material. School playgrounds, waterways, city streets, and parking lots, *SMH* are some of the places they have had to list in the code, but even these are written with caveats.
I have a Natures Head, and the material is composted with the rest of the sanctuary poo and bedding, and feeds the earth. I live in there high desert, so we are essentially making soil to enhance the burned out clay on basalt substrate on our property.