> It wasn't really fighting words. Yiannopoulos is smart and may have had some legal advice on top of that, so he never does anything that crosses the (legal) line.

Fighting words are not illegal (unless specifically prohibited by statute), they are just outside the Constitutional protection against regulation. So, you could engage in fighting words without crossing any legal line.

But, even if it is protected speech, freedom of speech means that private parties (like Twitter) are generally free to choose not to relay your speech, too.