I can understand not wanting to be referred to as CIS, it's hardly like everyone associates good connotations with the prefix considering the high tensions that can come in relevant discussions, but to outright ban it is absurd. To be clear,
Banning use of a word that’s part of our daily vocabulary around the world
it's not part of everyone's daily vocabulary around the world, and that is precisely the problem. I grew up in shithole Oklahoma, and the only time I have ever heard or said the word in-person is when talking with my trans friend, and I literally had to remember what the prefix was last time I was having a discussion involving it. I learned about the word not from someone who was politely explaining to me the difference, but from a very openly hostile person referring to CIS people as "scum" online. That's a really shitty first impression. I get that it's not actually an offensive term, but the feelings I was experiencing when I learned the meaning have sure stuck around and I don't generally enjoy using the term. If, instead, the term was more normalized, it'd just be a normal run-of-the-mill word to me that I wouldn't have a problem associating with.
I feel like banning it and calling it a "slur" is literally only problematic for these sorts of reasons and will lead to continued pointless division of something that should just be a basic descriptor. It's misleading, will associate negative emotions with the word, and will in turn hamper LGBTQ+ acceptance as the phrase is continually thrown around.
tl;dr Musk is a dumb fucking idiot that is going to harm society.