Its uses have own qualities that people find a tad annoying. There's also the rich, confusing ground that pops up when we aren't sure how the other person is using the terms - Are they saying, "What's new?" because they want to chat to you, or are they just saying hello?

What's up? tem o significado de 'Que foi?' no Reino Unido. Melhor dizer.... How's things? How's it going? Não usamos um artigo com a palavra 'slang'. A palavra 'slang' é incontável.

I take it as slang. If you said that to me, I'd think you were trying too hard to be cool, and wouldn't take you seriously. Still, may other speakers will accept it in a casual context.

Fxmaxey's answer is very good. In terms of writing, if you were to write a novel and trying to capture a "real" conversation what's up would be fine but in a more formal piece like an essay or news story you probably wouldn't use what's up as it is considered very informal speech. Also, if talking with someone of authority, like my boss or in power like a senator or judge, I would probably not use what's up but talking with friends and family I would use what's up.

It is informal speech, but I would hesitate to call it slang - at some point, slang moves to being an idiom. It is used to greet a person and ask about their current situation. It means "What's been happening with you lately?"