Cis and trans are both the same type of descriptor. If trans can be used as a slur, so can cis.
Not saying either are slurs by default. But they most certainly can be turned into ones.
Maybe we should clarify what a slur is? Because to my knowledge, a slur is a term that has such negative connotations that it is considered offensive and discriminatory against a certain group of people in itself, without any additional context. You simply do not use it unless you want to insult or offend someone from that group. If a term is only offensive based on how it’s used, it’s just a regular insult, not a slur.
So, “can be used as a slur” is not a thing. A word is either a slur, or it isn’t. Neither trans nor cis are slurs at the moment. I’ve never seen trans be used as an insult before. And even cis is almost never meant as a direct insult, merely as a reminder that someone is talking about things they have no lived experience with and should probably check their privilege. Yes, that can be in a demeaning way, but the goal there is not to hurt you, but to make you piss off. It’s an act of self protection. Nobody is seeking cis people out and starting to call them names unless they insert themselves into trans spaces and start talking shit about trans issues. If you’re doing that, and getting told off insults you or hurts your feelings, then, frankly, that’s a you problem.