It may have a different connotation in the US than in Europe. In US media I’ve only heard the term being used recently, in Europe it’s been around for decades.

In Europe, “Antifascist” is a label that people groups use to describe themselves, similar to how other describe themselves as patriot or pacifist. Someone carrying an Antifa flag at a protest is maybe comparable to someone carrying a Gadsden flag at a right wing rally.