Why Do New Planes Still Have Ashtrays Built In?

On any given flight, many passengers end up standing in the pristine, modern lavatory of a brand-new aircraft. It is a space defined by seamless surfaces, antimicrobial touchpoints, and mood lighting, sometimes even nicer than the bathroom at home. Yet, mounted on the door is a small, polished metal tray. It feels like a jarring relic from a smoky, long-gone era, but it is actually a piece of cutting-edge safety engineering. Why does this ghost of the past remain a requirement on the most advanced jets in the sky?