Funny because I've never seen it used as an insult, it's mainly used to clarify if a person is trans or not. I've never seen anyone shout "cis" in the street at non-trans people to insult them. I have however, seen and heard plenty of derogatory insults thrown at gay and trans people. "Groomer" for an example seems to be thrown around on Twitter on a daily basis.
https://www.dictionary.com/browse/cisgender
Despite what EM says, it's in the dictionary and has been for a long time, and no dictionary I've looked cisgender up in lists it as vulgar/offensive/slur. (Dictionaries always add a note next to offensive words.)
The Latin prefix "cis" (goes back a long time) means "on the same side as", and is used in many contexts besides gender.
IMHO you can't just announce "cisgender" is an offensive slur because you don't like the fact transpeople exist.