Yeeees, I do in fact think people should be "warned" (that is, told in advance that someone is trans so they do not unintentionally hurt said person) when someone has come out publicly. That's a big part of the reason people come out, to get acceptance of who they are from their community. If we want to swap specious arguments, I think the real question is why you think trans people should stay hidden and unrecognized?

Cara is a public figure, and she was quite famous back in the day. Coming out as trans hasn't erased her personal history, and unless we want to play a game of Celebrity Gender Reveal, the only way to tell people that this public figure goes by a new name and uses new pronouns is to use their old name. I'll concede the title is poorly worded, but just referencing a widely duplicated meme they were in once isn't sufficent to identify them.

And we really need out & proud trans celebrities. We need people who aren't ashamed of who they are and who they were to show the bigots of the world that we're not afraid of them, that we're not the weird boogymen they claim we are. To show scared trans kids what the hate they will doubtlessly face looks like and to show them that they do not have to face it alone. And if someone chooses to stay in the closet I will love and support them exactly the same as I will anyone else in the LGBT community, because that is an absolutely valid choice too.

You're lashing out because you're soundly losing an argument online, and that makes us all back up our opinions even more fervently than before. But please don't hurl hurtful accusations at someone else just to make yourself feel better. At worst you'll push someone ever closer to the edge of the Cliff of Bigotry, and at best you say this comical shit to someone thats been threatened, stalked, beaten, shot at, stabbed and endlessly teargassed fighting for queer & trans rights. And that doesn't really get a rise out of them, it just makes you look like a bit of a twerp.