I’m pretty confident the slur-status of a word is closely rated to whether or not it is used in a hateful way to refer to a people group or member of a group that is in some way disadvantaged. For example the n-word is obviously a slur and “cracker” or “whitey” obviously are not. That’s why cisgender isn’t a slur, even if people can and have use(d) it in a hateful way.