Good point. I just checked our company's web analytics and IE makes up about 2% of web traffic. So it's not dead yet, but I guess you can make a case for not supporting it.
I thought CSS variables were still not fully supported enough to use in production. I know you can use a polyfill, but that seems to detract from the minimalist approach of these basic CSS frameworks: https://caniuse.com/#feat=css-variables