After some additional thought, I think this is the key test: clearly there are times that people say neutral/nice stuff about the cisgendered. And apparently there are times when people say mean things about the cisgendered (although so far a grand total of one example has been actually cited in this thread, and I’m far from certain that it wasn’t mean ironically). (And no, people talking about white heterosexual cisgendered privilege doesn’t count unless you also think that makes “white” a derogatory term).
Are people saying neutral/nice things more likely to use a term other than “cis” or “cisgendered”, while people saying mean things are more likely to use “cis” or “cisgendered”? If not, how can you argue that it’s an offensive or insulting term?
I suppose there’s one other possibility, which is that a term just outlives its relevance. For instance, “Oriental” or “Jewess”, where we all know that society used to have prejudices, and a term that seems to be from yesteryear carries the aura of those prejudices with it, even without any obvious insulting implications. But that can’t possibly apply in this case, because of course there is no historical period of past prejudicial attitudes towards the cisgendered.