A useful rebuttal in LGBTQ Nation explained why “cis is a slur” is total bullshit: People who say that cis is a slur don’t offer an alternative term to use that is non-offensive. Because they don’t offer alternative inoffensive terminology, as there is with all other slurs, it’s clear it’s not the word cis they’re objecting to, but the existence of any words to describe the fact that some people are transgender, and some people aren’t.
If you were to ask one of these morons, I bet they'd say that the alternative non-offensive term is "normal". "Normal" is a safe and reassuring blanket that tells them that they don't have to change, that they're in the right, and that all these other people are abnormal deviants.
The other portion of it is that they themselves didn't choose the name "cis" and so they feel as if they are being labeled, and labeling is what they do to others to subjugate and humiliate them as abnormal, so that's how they feel now that they are the ones being labeled.