It's used as an art tag rather colloquially on adult websites that host such content. It's a Japanese word for individuals with multiple types of genitalia. I haven't really seen it used as a slur, it's a search term for finding something many people enjoy seeing.
We can't let the haters steal word meanings away, especially when there's plenty of perfectly decent people using them to describe something they appreciate. I'm guessing that's exactly the sort of thing that those who would use futa as an insult would aim to accomplish.