I have a feeling that the people that think "cis" is a slur just don't understand what it means, or are outright transphobic.
I recently had a guy admit he might be transphobic because he "prefers actual women", and later uttered the sentence "maybe women don't like to be called 'cis women'?" and like, dude, it's just an adjective that means the opposite of trans. But how dare we imply that trans women are women?!
In the end, treating cis / cisgender as a slur is yet another way to attack trans people.
When you say "trans woman" you affirm that they are women, and trans is just an adjective. When you say "transwoman" it can imply that they are something different altogether, and TERFs have certainly used it as such. Like, I dunno, a carpark isn't a park? That's the first example that came to mind, anyway.