People think ‘cis’ is a slur because that's how they think about and use ‘trans‘, as a slur.
@FuchsiaShock That's a possibility, too.
I can't pretend to know what's in the minds of other cisgendered people, but I can say that, for my part, I treat others the way they treat me.
People reap what they sow, with me. If they treat me like they want me to treat them we'll get along fine, or not, depending.