It's a type of electronic dance music. I had to google it as I hadn't heard the term myself.

It's slang... it can mean different things. For example, Will Smith and Jada are still a thing?.. it could mean, they are still together? or they are still relevant? .. but it your case it means " people are still listening to that?"

"is ... still a thing" simply means is the subject (dubstep in this case) still popular? People often uses phrases like "I'm into..." or "... is my thing" to show an interest in something; so this is a variation on the latter of these two phrases, to ask about the current general interest in something.