Soaking is not enough to remove phytohaemagglutinin. Phytohaemagglutinin is destroyed by sufficient cooking, which according to the FDA's food safety book[0] means boiling for 30 minutes. Slow cookers do not get hot enough. And it's not just kidney beans; many other beans also contain phytohaemagglutinin, sometimes at dangerous levels.
It's not a culinary requirement, it just means you don't have to boil them for absolutely ever. A pressure cooker is quite effective if one happens to have one.
My experience with most beans-qua-beans is that a pre-soak or pressure is needed or they'll still be somewhat chalky even after 90+ minutes at a boil. Lentils and peas don't have this limitation.
[1] https://www.seriouseats.com/2014/09/soaking-black-beans-faq....